Put blue food coloring in the milk.
A Seuss of a Shocker
Green eggs and bacon is a twist on Dr. Seuss.
Wrap a rubber band around the kitchen sink sprayer so whoever turns on the faucet gets a big, wet surprise.
Place a big ol' fake roach in a family member's slipper.
Tape a dollar or a quarter to the floor in a high-traffic area, and watch the parade of clowns bend down and unsuccessfully try to pick it up.
Alarming Alarm Clock
Set someone's alarm clock an hour early. Or set a bunch of clocks to go off at the same time, and hide them around the room for a noisy wake-up call.
Confuse callers by leaving the following outgoing message on your answering machine: "Hello? [pause] Hello? [pause] Is anyone there? [pause] April Fools'!"
Insert a raisin into a tube of toothpaste. The unsuspecting person who squeezes it will be dismayed by what comes out onto his toothbrush.
A Salty Surprise
Salt in the sugar bowl is a classic switcheroo.
Infested Ice Cubes
Pop some plastic ice cubes (the ones with toy flies or ants inside) into your guests' drinks, and watch their expressions go from thirst to horror.
A mashed-potatoes-and-gravy "sundae" is a good gag for the afternoon, when everyone thinks the jokes are over.