Okay, if you want to get ripping drunk (and really, who doesn't go to Mexico and do this at least once?), then I recommend the Vamanos tour. Once or twice a week, the tour goes out to all the resorts, picks up about 50 twenty- and thirty-somethings, and carts them into downtown PV. On the way, you are told the situation: you will be visiting three bars that evening, and you will be drinking a lot. You are handed a yard-arm drinking cup (literally a yard tall).
Your first stop is Senor Frog's. Your yard arm can be filled with anything for about $5 (I had a yard of Daquiri). After one, you will be having a great time! You spend about 45 mins to 1 hour there, and then you are herded onto the bus for the next club.
You arrive at Carlos O'Brians. Here, you get your yard cup filled for free (heaven help you!). You can then choose to stay at COB, or head down the street to your final destination.
We left COB, and went to the Zoo. It was absolutely packed with fun-loving but very drunk tourists. A very fun time was had by all.
If you are staying at a resort, ask your tour coordinator about the Vamanos tour, they will be able to sign you up for it. If you are travelling independently, call in to one of the resorts, or drop by and see if you can sign up with one of the tour coordinators. Cost is about $50 USD for the whole evening.
Dress Code: Anything goes.