Jungle Tour:
There's not really a jungle. You get on an old skol jet ski and go through the mangroves, then go snorkeling. Really, it's a snorkeling excursion. Expect to pay $35 to $50 for it. The lobby at your hotel will have a Mexican that can hook you up.

Tulum:
Do this one. It's the one with the Mayan Pyramids. Most spring breakers are too cheap to spend the $70-$90 for this full day tour...but you should. The ruins sit on the possibly the best beach in the entire world (part of the reason they may have been built there.).

Cozumel:
Only if you are a hard core snorkel or scuba diver should you bother with this full 12 hour tour. $150-$250 with snorkeling, about $60 more for scuba gear.

Fishing:
Is expensive, but INCREDIBLE. If you have a large group, it can be less money to do it. Expect to pay $60-$200 per person to go for the boat, equipment, and some beer.

Isla Mujeres:
This is really a great little day excursion. You can hook up a ferry over to the island (only takes about 30 minutes to get over) by going to the little tour guy at your hotel by the front desk.

It's small there, and you can rent snorkeling equipment right on the beach.

Jet Skiing:
You can rent equipment in front of most hotels. $60-$80 for a half hour. Check the equipment over though, or you could get charged if something was broken when you bring it back.

Parasailing:
Hook this up at AquaWorld, across the street basically from the Omni, down by the Oasis Cancun.

Swim with the Dolphins:
Again, ask at your hotel front desk. There are two to choose from, the 'pet the dolphins" - which sucks, and the "interactive" which means they drag your sorry ass around the pool and play with you in the water.