Practical jokes that can easily be done.
1) Dress up as the Grim Reaper (including scythe), creep into your victims bedroom in the middle of the night and wake them up bellowing 'YOUR TIME HAS COME'. Just as they are about to have a heart attack whip the hood down and have a jolly good laugh at their expense.
2) Glue someones front door shut. Nobody gets in or out. How good would that be?
3) Resign somebody from their job while they are on holiday.
4) Creep up behind someone and wire their ears up to the mains.
5) Put on a balaclava, burst into the home of someone you know brandishing a knife, pretend that you're a crazed rapist and only pull the plug when they get really upset.
6) Wait until your best friend is going on holiday abroad then hide weapons or fake bombs in his or her luggage.