Dip their tooth brushes in salt, make the door knob extra slippery with lotion so they'll have difficulty using it, steal all their clean clothes and towels and store them somewhere when they're taking a shower, or dip their hands in water while they're asleep to make them wet their beds. Or do the pie in the face thing by ultimate surprise, or when they're asleep. I did this terrible thing at my eighth's birthday though, where I mixed up a coca cola/sprite concoction with salt, sugar, ketchup, bits of raw potato and marshmallows, tap water, mustard, a little droplet of soysauce, and let this kid drink it. He actually liked it! But then when the other girl told him what the contents were he turned green and puked it all up. Slight regret for what happened that time, but he didn't hold it against me so I think we're alright. XD You could try that! But with less extreme... ingredients.
The ice cube mentos idea doesn't usually work. Most people will drink the whole soda before the ice cubes melt.
Do that American pie shit and fap into a bottle of lotion and let them rub it all over... Wait but you gotta pee in it and mix some poop in it
1.Use red dye tablets in their shower head 2. Replace their salt with sugar 3. If they are scared of spiders or any small creature put it in their bed before they wake up 4. Print out disembodied pictures all over their bathroom walls before they get up. 5. Put jelly and butter on the doorknobs 6. Soap up the floors in the bathroom and bedroom