If your milk comes in a cardboard container, add a few drops of food coloring. It's harmless April Fool's joke but the results are pretty colorful.
This April Fool's practical joke is old but it still works. Superglue some coins to the sidewalk or any spot that has a lot of people walking around. Make sure it's an appropriate place, then watch people break fingernails to get the coins.
The top 10 jokes for April Fools 2011 thus far: HERE