Make that two alarm clocks. This is one occasion where lateness will not be forgotten. Appropriate wedding attire. Contact the wedding planners as early as possible to get the necessary wardrobe parameters. String and cans for the getaway car.
Tin cans are the stereotype, but aluminum soda cans are just as noisy and easier to attach.
You'll be giving speeches, throwing parties and who knows what else. It may be their Big Day, but you'll definitely have your chance to shine. It's not like you're marrying the guy, but you ARE making a commitment, and you'll be expected to fulfill a number of duties that extend beyond the big day. Remember that guests are likely to forgive a nervous groom if he walks down the aisle with his shirt untucked, but they'll wonder what kind of friend YOU are.