I went to Cancun 2 years ago for spring break and it was the craziest time. I thought I was the only one who didn't like Senor Frogs. The night that we had gone there, I had stayed up 2 days straight, so I was pretty dead. I was actually falling asleep while I was there... my friends took a picture of me standing there as my eyes were closing. Needless to say, didn't like it there. I had the best time at Pat O'Brien's Mardi Gras party even though I lost my friends after being there for only a half hour. It didn't matter though because I ended up staying there way after the party ended just meeting random people. My two favorite clubs were La Boom and Coco Bongo. My friends and I wanted to actually do day activities, like go snorkeling or boating, but when it came down to actually setting up stuff, we got lazy and just headed to the beach.
And when you run out of places to go eat, there's always Outback. We stayed at Continental Villas Plaza (Right across from Pat O'Briens and Outback) and ate there after getting sick of the crappy meals from the all-inclusive plan.
As for luggage, I think I managed to fit all my clothes into my carry on bag. I had heard all the horror stories about people's luggage getting lost and didn't want to take the chance of that happening to me. Definitely packing light is the best way to go... besides it will be warm there, you don't need to be wearing THAT much clothes.

Have lots of fun and don't drink the water!