For the office or work place:
Take in some snacks now in a plain brown paper bag, leave it on the table for folks to help themselves.
Then on April 1 you bring in the bag, leave it at the same place, roll the bag down a little so it's not open. Inside the bag taped to the bottom you put a note. "Why are you looking in here? Is this your bag?"
My dh did this at his office where everyone is a chow hound so it went over really well.
For kids lunches. When I was in school everyone took a lunch with a sandwich. One year this guy's mother put a piece of cardboard in his sandwich instead of the cheese. She had "April Fool's" written on the cardboard. She was nice enough to put a piece of cheese in the bottom of the lunch sack.
I always left a message for my boss on April Fool's Day that I had fallen off my horse and broken my leg so I won't be in. It was so funny because each year they would have forgotten I'd done it the year before.