Poppins decides to become one of Grandpa's dreamers. The reference is to the lilies of the field, which Biblically do not sow or reap, but are cared for by God. Dreamers don't worry where their next meal is coming from—though the truth is that Poppins' toys seem like they could maybe make him some money, if he wanted to sell them. But of course he doesn't need to because Grandpa has an infinite supply of money, for some mystical reason. Infinite supplies of mystical money make dreaming a lot easier.
TONY: We can't just pop out of the building with no place to go. Very bad idea that, you know. I know two people that did that once—they went out of the building, they were uncertain, so they just walked and walked and walked and finally they just died... of hunger. Now you wouldn't want anything like that to happen... because if, if that happened... you're so beautiful.
You Can't Take It With You is into dreaming. But the film also endorses impulsive goofiness and setting off without figuring out where you're going to dinner. The film sees dreamers as folks without plans, perhaps. Tony here is maybe parodying his father a little; Mr. Kirby would never set out for dinner without knowing where he's going (though, as Tony shows later, deliberately showing up on the wrong night for dinner to thwart your sweetie's plans can have some disastrous consequences).
TONY: It takes courage. You know everybody's afraid to live.
ALICE: You ought to hear Grandpa on that subject. You know he says most people nowadays are run by fear. Fear of what they eat, fear of what they drink, fear of their jobs, their future, fear of their health. They're scared to save money, and they're scared to spend it. You know what his pet aversion is? The people who commercialize on fear, you know they scare you to death so they can sell you something you don't need.
Alice's advice here (taken from Grandpa) seems like a variation on Franklin Roosevelt's famous comment that "there is nothing to fear but fear itself." Was Roosevelt right? The fact that Tony and Alice are talking about people "nowadays" is important. The film was released in 1938, during the Great Depression.
People were very afraid of taking risks then, and with reason; there were few jobs, and making a living was extremely difficult. In You Can't Take It With You, the only thing keeping you from your dreams is fear. But that's not exactly always true in the real world, outside the film.
GRANDPA VANDERHOFF: How's Essie doing?
KOLENKHOV: Confidentially, she stinks.
GRANDPA VANDERHOFF: Oh well, as long as she's happy.
Essie isn't actually a good dancer, but she likes dancing, and dreaming of being a dancer. That's good enough for Grandpa.
GRANDPA VANDERHOF: [offering grace] Quiet, please, quiet! Well, sir, here we are again. We've had quite a time of it lately, but it seems that the worst of it is over. Course, the fireworks all blew up, but we can't very well blame that on you. Anyway, everything's turned out fine, as it usually does. Alice is going to marry Tony; Mr. Kirby, who's turned out to be a very good egg, sold us back our house—he'll probably forget all about big deals for a while. Nobody on our block has to move and, with the right handling, I think we can even thaw out Mrs. Kirby here. We've all got our health; as far as anything else is concerned, we still leave that up to you. Thank you. Bring it on, Rheba!
The conclusion of the film is Grandpa offering up a prayer, in which he basically says he's not going to plan anything and will leave it all up to God. You can pursue your dreams because God will take care of you. Which is a comforting thought, and works in the film, anyway.