when you get to the hotel someone distract the groom...
while the groom is distracted get his toothbrush and stick it up your bum... take a photo with the gooms camera (film preferably) then replace the camera and toothbrush where you found them.
once this is done dont play any pranks on him... he will know what you did when he develops the photo's ;o)
Done a similar thing to a mate that got so drunk he passed out one night, we got a load of makeup and made him up to look like a cheap hooker took a few photos of him on his own camera and the carefully cleaned all the makeup off.
I got called a few choice names when he got the film developed.
Take him to a lapdancing club and order a private lapdance for him allowing him to see the cutie who will be doing him. Then blindfold him, replace the cutie with a well muscled, well oiled and most obviously gay guy! If you can get the guy in drag that would be even better. Take lots of photographs as you egg the stag on!! One for the wedding album
And what are you going to do if your nasty pranks go wrong and the groom is seriously hurt or injured in some way and the wedding has to be postponed? Why can't you just have a few drinks and a pleasant night out.? Don't want to be a spoilsport but the stress and run-up to most peoples' wedding is difficult enough to get through without having to cope with juvenile antics.