Can you be more specific? Smartest how? Like, "smartest to entertain myself," "smartest to irritate other people," etc. Because you can buy a lot of crickets online for twenty bucks and then release them into whatever building(s) you want... (Same with ladybugs!) Edit: I think technically the smartest thing would be to invest it, but that's boring, right?
Actually: you invest a few grand, you put a few hundred into savings, and you put quarters in the parking meter or the washing machine. There is a specific set of jobs that the Yuppie Food Stamp does well. Twenty bucks has the face of President Jackson, the man that gave us the Trail of Tears and political patronage. That bill symbolizes a lot of greased palms and greased wheels. It's still three hours of pay for a lot of people but half an hour's pay for others. It's a large pizza and a couple liters of soda, it's a replacement USB keyboard, it's a week's worth of the good frozen burritos and a couple pounds of carrots. ATMs give us twenties: they're the fundamental spending unit. It's never a real problem to break a twenty the way it can be to break a fifty. If you still have one in your wallet, you aren't so screwed that you need to stop and get more. If you give a twenty to a friend, you'll never mind not getting it back but you're not a skinflint either.
I'm not quite sure, investing seems obvious, but there could "fun" ways to spend it. I would say I'm quite partial to thenewgreen's suggestion below.
Honestly, "investing" $20 isn't going to do much. Ever. But, nobody can ever take away the day you saw 3 movies in the same day and still had change from your twenty for food!