+1 on all the above. Everyone pretty much covered the high points.
A few additional morsels:
Cancun is best AVOIDED during spring break and hurricane season.
Not all hotels/resorts are created equal WRT quality/service/beach access/etc..
If you want to stay in Cancun, I would recommend one of the hotels on Kukulcan Blvd. Personally, I like the Gran Melia. If all-inclusive is your thing, I would recommend the Paradisus Riviera in Puerto Morelos (half-way between Cancun & PdC).
If so inclined, stop by the cigar shop in Kukulcan Plaza for a few "island" sticks (Cuban cigars). All other places you are probably getting fakes.
If you are into fly-fishing, Ascension Bay, Bocapaila, & Isla Blanca are the preferred hot-spots for Bonefish & Tarpon.
