If you keep the same dressing room overnight, move everyone's gear around so when they get dressed they have all the wrong stuff.
One guy I played with once unrolled condoms over the ends of everyone's sticks.
Most guys have expandable helmets, make them all bigger, or smaller if they have big heads.
Again if you can get at the gear after everyone is gone, take their skates, tie them all together and make a pile of skates in the middle of the room.
Spiderweb the room in tape. Ceiling to floor strands, wall to wall, wall to ceiling etc... so at the end it looks like the motion sensor lasers in movies and you have to duck and jump to get through. If you have a good group they'll try to see how long it stays up.
Freeze someone's jock into a block of ice and make them dig it out before the game.
Get the P.A. announcer to use different names (female/european/or like Don Cherry) instead of what's on the game sheet.
Take one sock from every player, if conditions permit you can then freeze them like the jock.
Swap out the pre-game music for something ridiculous, personally I like to replace it with big band era stuff from the 30s/40s because nobody knows what to think of it.
One of my favourites: Taking down our jerseys hung up before the game and replacing them with an identical set about 2-3 sizes smaller (like hang up peewee shirts and take down midget ones), everyone thinks they got stuck with the small shirt until they realize everyone else is having the same problem.
just dont do anything that will potentially injure or illegal 
