I can think of some very funny ways to play practical jokes on people… especially those with memory loss.
1) Rearrange the kitchen; having a kitchen fire is optional.
2) While they are out of town, close one of the neighborhood’s two grocery stores… have the other rearrange the aisles.
3) Randomly ask “Do you remember ‘this’ from last night?” (ice cream, movie, whatever) regardless of whether or not it actually occurred.