Off the top of my head:
1. Must order, and drink a Babycham ... preferably with a cocktail umbrella and/or cherry in.
2. Must always announce, loudly, when he is going to the toilet.
3. He has to dance between venues, rather than just, say, walking.
4. Give him a list of items he must collect before the end of the evening. I'd suggest than an item of female underwear is on the list
5. If anybody outside the group asks him any question about his forthcoming wedding he must make it clear that it's a civil union to a man called Derek.
6. Should any female, of any age or appearance, talk to him he must explain that he can't buy them a drink, as he's getting married, but offer to share his current drink with her.
7. He has to 'recognise' an old school friend in bar and try to get them to remember their (improbable) shool-day adventures together.
8. If you see anybody who looks like a Hell's Angel he must punch them in the back of the head and shout, "Puff!". Or perhaps not
I'd think of some more, but I need to go get a sandwich.