By good angels tenanted, (2)
This line kicks off the generally happy vibe at the start of this poem. Any place where good angels would live has to be a happy spot, right? It also fits with the supernatural themes of this poem—there are spirits everywhere in and around this palace. At the same time, we never actually hear anything else about those angels, so it could be that this description is metaphorical.
And every gentle air that dallied,In that sweet day, (13-14)
This line makes us think of the most beautiful summer day. Think of the best day you've ever had at the beach, when it was warm, but not hot, when it was breezy but not windy—just like that. The speaker's emphasis on the "gentle air" and the "sweet day" really helps to bring that idea home.
Wanderers in that happy valley, (17)
Now the speaker just comes out and straight-up tells us that this valley is a happy place. He even reinforces that feeling by calling the people who are heading through the valley "wanderers." They're not in a rush. They aren't going straight from point A to point B as fast as possible. They're wandering, just like happy people would.