it's spring. there are some good parks in brooklyn. walk over, get some fresh air, find some cafe, get a bite on your way back, or maybe pick up like one ingredient to make dinner with, something like that.
if you wander by some bowling alley or arcade or karaoke club, pop in. Personally, my goal there would be to have a fun time with strangers, but you could just do your own thing.
if you're in manhattan, go up to the bar on the roof of some hotel, don't even bother getting a drink, just appreciate their view. or get a drink, they're expensive but often tasty.
wander. the city is dense and walkable.