This article by Holly Gramazio suggests several ideas for games you can play alone or with friends while in public. My favorite of the lot was posted by a Redditor, and seems like a great way to feel more positive about the world:
When I'm out in public or driving or something, I like to pick a person within my sight and imagine how they're somebody's FAVOURITE person, and if that somebody could be where I am in that moment, they'd be overjoyed to see them. I imagine how that person might joke around, or I think about the nice things they might do for their friends or family, and I wonder if there's someone across the country that wishes more than anything that they could be as near to that stranger as I am now.
(My partner and I have a new, stupid game that makes us smile–when we're watching sports together, we add an "N" to players' last names.)