My April Fools joke just ended last night. On April 1st (really, kind of a coincidence) my wife brought home some groceries, including her weakness, oreos, which set my plan into action. The next day, I went out and bought a bag of oreos and hid them. Once she had eaten about 10 or so, I started replacing some every day. The bag slowly went from 3/4ths, to 2/3rds, to 1/2, to about 1/3. I, of course, didn't replace the exact amount, or it would be too obvious, so the bag slowly diminished.
I've been able to tell the past couple of weeks that she has been looking at the bag funny, but just kind of shrugged it off, I guess. Anyways, last night, she said to me, "Aren't you proud of me? I've had this bag of oreos for like, 3 or 4 weeks, and it's still not gone." I bursted out laughing and had to tell her. She took it in stride, and then realized that what she got out of it was more oreos.
Not sure how good the joke sounds on paper, but I certainly had fun with it, and it was certainly a harmless to play on someone you love.