The Hair Dye prank is a good one because most of the time it won’t get you in trouble with the grooms missus, but it will get a lot of laughs.
You can get spray cans of hair dye colour in most fancy dress shops. Just pick a colour, the more ridiculous the better and when the groom goes to sleep, give his head a good covering
When he wakes the next day and eventually spots his hair is blue, there’s be a good old laugh to help ease those hangovers.
Make sure and buy the stuff that washes out after a day. But you don’t have to tell him that, just keep him away from the showers and washing his hair is the only trick.