This is my kind of activity. Not a prank, really, but a dash of shenanigans. Posting funny or thought-provoking things in the middle of the mundane creates a sense of paying it forward with a moment meant for someone else. How very lovely.
I love to place the humor and then watch from an undisclosed location nearby. I once left a gnome peeking out from the bushes in the Ducal palace gardens in Florence and then watched from several benches over as people walked by, did a double take at the gnome, and then would point and laugh. I so enjoyed watching people walk by and smile at the little friend, Norman, pictured below:
I realize that because this is a picture of a picture, that this just looks like a creepy Sasquatch-looking gnome photo, so you will just have to trust me that it was cute and not horrifying.
Try it, it is very fun! Do it with your friends, or kids. Or you can really go for the gold and make a city scavenger hunt where you go around your city and do all sorts of fun activities like a tourist in your own city. No matter what, though, have fun and pay it forward.