"Their" is used meaning your brothers/sisters/moms/dads and is mostly directed at meaning your parents.
1) Hide their coffee/tea (whichever one they drink to wake up).
2) dye some ice cubes an odd color (with food coloring ONLY), freeze them, and put them on top of all the ice cube trays so they think their drink's gone bad.
3) For cassette player/Rio hide batteries, "lose" their place, "lose" their headphones (not best to do this if they're right in the middle of a long OTR/Old Time Radio show... I know from experience x_x).
4) Stuff their favorite shoes with fake craft feathers or pony beads.
5) Hide a life-sized doll under blankets so that they yell at it thinking it's you (they're sleepy in the morning if you've hidden their coffee... o.O)
6) surprise them with an evelope popper (I got one at a library book fair, I doubt that'll be your source) popping out of an empty packet of garden seeds or an old envelope from a bill.
7) E-mail Surprise - If they check the e-mail in the morning, turn the speakers up to loud (IMPORTANT: NOT ALL THE WAY) and send them a musical e-card... hee hee.
8) Rubber-band the sink spray and rig it so that when they turn on the sink it sprays them. You must have the rubber band pressing on the sprayer handle. De-activate if mom's wearing her best clothes or dad's late for work. (I've used this one for three years... poor Mum, she forgets when it's April first....)
9) Use all the hot water in the shower or turn some taps on while they're in the shower, but not for a long time. 20-30 seconds best. Don't do this if their already steaming at the ears because of your pranks.
10) Switch their dresser drawers around.
11) Set their clock back an hour or two... NOT ON WEEKDAYS... or set them forward.
12) Put a weight-activated doormat (like the ones that cackle for Halloween) under their sheet to 'lull them to sleep'.
13) Fill all of the sinks with water, then put rubber duckies into them and the toilets. (submitted by Kyra Hawkens).
14) Turn pictures upside-down in their frames. (submitted by Divine)
15) Glue a card together and give it to them. (submitted by Divine)
16) Switch the bags around in cereal boxes (open up the Cheerios and out comes Lucky Charms!). (submitted by Divine)
17) Stuff paper into shoes so that when people put them on they'll think they outgrew them. (submitted by Divine)
18) Put rice or sprinkles in their bed. (submitted by American Angel)
19) Put nail glue (that always works the best - i know from experience :P) on their seat. If they're planning to sit there a while IT WILL STICK. (submitted by American Angel)
20) If you know their instant messager password for msn, change their nickname to something else.
21) Super glue a quarter to the sidewalk and wait for people to come by. (submitted by Josh M)
22) Get eyeliner (make sure it is not waterproof) while they are sleeping gently write on their faces with it. When they wake up watch then as they realize what has happened to their face. (submitted by Becca)