I think pranks that are at peoples' expense are not funny and have been done to death, risen from the grave, and then done to death again.
If I were to do public pranks, it would be something nice, or just strange, but not designed to bother people.
Random idea that just occurred to me:
Write "I don't want it" on the back of bunch of dollar bills (like 10). Go out and hand people the dollar face up and say "Here you go, have a dollar" and then after they react, say "flip it over, what does it say?" and when they say "I don't want it" go "oh okay" and start to reach for it.
Don't fight them for the dollar back, just go with however they react.
Not a brilliant idea but it took me 30 seconds to think up. Think of ideas that are clever and/or friendly.
Good luck!