Large hard cooler in a trailer. Just bungee it on and start pedaling. Pros: You likely already own the cooler, and a BOB is always handy to have around. Good insulation. Toss the empties inside. Cons: Lots of space for your friends to stash their beer; expect a heavy load. Hard to access with bungee cords across the lid.
Pony keg in a BOB trailer. Set the barrel on bags of ice, and bungee a couple more bags around the sides and top. Strap carpet samples over the ice for insulation. Remember to bring cups. Pros: plenty of beer for everyone (approximately 80 12-ounce servings). Cons: depending on your terrain, 90 pounds of beer sloshing around can create balance issues on your bike. It can also create flat beer, which means you should drink really fast (maybe this is a pro?).