Phoenixes are magical birds that age, die, and are reborn from their own ashes. They symbolize endurance and new beginnings. Professor Dumbledore has an actual phoenix, Fawkes, who swallows an Avada Kedavra meant for Harry, dies, and returns as a "tiny, ugly, featherless" (37.20) chick at the end of Book 5. Fawkes the phoenix has saved Harry's life before: his tears heal Harry from the basilisk bite in Book 2, and he helps to soothe Harry's shock after the confrontation with Voldemort and the death of Cedric Diggory in Book 4. As such, Fawkes is a recurring character in the novel; he is endlessly loyal to both Harry and Professor Dumbledore, and he seems generally aligned to the cause of good.
Fawkes also seems symbolically connected to Harry's emotional rebirth at the end of Book 5. Indeed, Harry is something of a phoenix, given that he was found living in the ruins of his parents' old house when Voldemort tried to kill him in the first place. He has also nearly died and risen again several times: after his showdown with Professor Quirrell, with the basilisk, and with Voldemort himself at the end of Book 4. In this case, however, Harry's rebirth is more metaphorical. When Professor Dumbledore meets with Harry in the second-to-last chapter of the novel, Harry finally vents all of his frustration at the situation in which he's been placed. It's like the lancing of a wound, and all of the dark resentment and suspicion against Professor Dumbledore that he has been bottling up comes spilling out.
By the end of the meeting, Harry has come to some kind of peace that soothes the violence of his grief and bitterness. He begins to return to the Harry Potter we know and love: older and wiser, perhaps, but less gloomy and angry than he has been throughout Book 5. Accompanying Harry's own emotional rebirth is Fawkes's physical rebirth: Fawkes took the Killing Curse meant for Harry and returns to a venerable, ugly chick. Harry has been broken down emotionally and is taking the first steps to learning to trust Professor Dumbledore and his friends again.
The Ministry's Golden Statues
At the front of the Ministry of Magic is a set of five golden figures:
Halfway down the hall was a fountain. A group of golden statues, larger than life-size, stood in the middle of a circular pool. Tallest of them all was a noble-looking wizard with his wand pointing straight up in the air. Grouped around him were a beautiful witch, a centaur, a goblin and a house-elf. The last three were all looking adoringly up at the witch and wizard. Glittering jets of water were flying from the ends of their wands, the point of the centaur's arrow, the tip of the goblin's hat and each of the house-elf's ears, so that the tinkling hiss of falling water was added to the pops and cracks of the Apparators and the clatter of footsteps as hundreds of witches and wizards, most of whom were wearing glum, early-morning looks, strode towards a set of golden gates at the far end of the hall. (7.66)
This statue at the front of the Ministry of Magic demonstrates the Ministry's official opinion on the status of the different magical creatures in the wizarding world. At the center is a "noble-looking wizard" with a beautiful witch next to him. Then, next to these magical folk are three creatures looking "adoringly up." Now, a centaur is half-human, half-horse, so how a centaur could look up at a wizard and a witch, we don't know.
This condescending view of house-elves, centaurs, and goblins – that they must all look up to magical humans – underlines Bill Weasley's suspicions in Chapter 5 that the goblins, fed up with mainstream wizard prejudice, might join Voldemort out of desperation. What's more, this statue also suggests that Professor Umbridge's prejudices about half-humans – and perhaps about all magical creatures – is more widespread in the magical community than we might like to think.
When Professor Dumbledore tears this fountain apart to protect Harry from Voldemort's Killing Curses at the end of the novel, he's using the tools that are near at hand. But there is also a symbolic value to Dumbledore's destruction of these statues: Dumbledore is willing to use whatever tools necessary – centaurs, house-elves, whatever – to fight against Voldemort. In this struggle, wizarding snobbery about other magical creatures can only get in the way of achieving their goals. Professor Dumbledore doesn't support the values that these golden statues represent, so no wonder he is so willing to destroy them.
Mrs. Black's Portrait
All of Number Twelve, Grimmauld Place seems designed to remind Sirius of the dark years of his childhood, with its gloomy, pest-ridden rooms and the Black family tree with his name burned off. But the one object that really drives home exactly how much of an outsider Sirius is to his own family is the portrait of his mother that hangs in the entrance hall of Number Twelve.
Every time someone makes too much noise or disturbs the portrait in any way, the painted Mrs. Black will start shrieking catchphrases the real Mrs. Black enjoyed: nonsense about blood traitors and filth that clearly gets to Sirius. Mrs. Black's portrait is not a fully realized character, since her responses seem automatic and not complex. Instead, her constant shrieking if anyone in the house speaks too loud underlines how trapped Sirius is at Number Twelve. If he moves a muscle out of turn, the house itself starts to yell abuse at him. No wonder he goes stir crazy over the course of a year imprisoned there.
The Two-Way Mirror and the Knife to Open Any Lock
Sirius gives Harry two extremely useful gifts over the course of the novel (though Harry only opens the two-way mirror once Sirius has died and it's too late to be helpful). Harry does get a lot of use out of the knife that opens any lock. Sirius's joy in giving Harry presents – and the extremely useful nature of these gifts – shows exactly the problem that Sirius's character presents to the narrative arc of the novels as a whole.
Rowling has said that leaving Sirius alive would give Harry too much help on his road to facing Voldemort: "it is more satisfying I think for the reader if the hero [Harry] has to go on alone and to give him too much support [from Sirius] makes his job too easy, sorry" (source). It sounds cruel to say this, but Sirius's love for Harry appears to be why Rowling felt he had to die: too much help from a powerful adult wizard like Sirius would hamper Harry's ability to stand on his own two feet by Book 7.
These presents prove that Sirius is willing to assist Harry, possibly to the detriment of Harry's own character development. But as symbols of Sirius's love, they also draw out the tragedy of his death all the more. When Harry looks at the two-way mirror and cannot find him after Sirius has gone through the archway, we start to tear up a little. Sirius gave Harry these presents because he loves him, and now we have proof that that source of support has been brutally taken from Harry.
The Department of Mysteries
The Department of Mysteries is the part of the Ministry of Magic where they study the great, well, mysteries of life. Ron stumbles unluckily into a room full of brains in jars: a place to analyze the human mind. A Death Eater winds up with a crystal bell jar on his head, a jar that de-ages his head to babyhood and then re-ages it back to adulthood over and over. The jar contains time itself. Then, of course, there is also the veiled archway that kills Sirius. This archway leads directly to death, and once you go through it, there's no turning back. No one knows what is on the other side, though Luna Lovegood and Harry can both hear voices coming through the archway. Perhaps the dead are just out of sight, but still present somehow beyond the archway.
So, there is the mind, time, and death. What is going on with that locked door that cannot be opened, the door that melts Harry's magical lock-picking knife? Professor Dumbledore tells us that this room contains:
... a force that is at once more wonderful and more terrible than death, than human intelligence, than the forces of nature. It is also, perhaps, the most mysterious of the many subjects for study that reside there. It is the power held within that room that you possess in such quantities and which Voldemort has not at all. (36.212)
This power appears to be the weapon Harry holds to protect himself against Voldemort: the power of his own heart. It's essential to the value system of the Harry Potter novels to know that Rowling considers the heart to be stronger than any other force, including the brain, time, even death. Harry is a hero not because he has extraordinary smarts or talent (though he has a good amount of both) but because he has a vast capacity for love and compassion.