Wife and I went to Cancun (actually Puerto Morelos which is a bit south, in between Playa Del Carmen and Cancun) for our first anniversary. We stayed at Now Jade which is part of AM Resorts. All-inclusive, great beach, decent hotel restaurants. We took a tour out to the Chichen Itza ruins in the middle of peninsula. Decision was between this and Tulum, and wife had always wanted to see the big one. The actual ruins themselves were awesome, but the bus drive is like 3-4 hours each way, and we went in July so it was like 105 degrees with 70% humidity. Take lots of water and sun screen.
On another visit to Cancun back in 2004-2005 with some college buddies, we stayed in Cancun proper at The Westin resort. It was waaaaaaaaay far south compared to the rest of the hotels on the strip, so bus rides took 20 minutes just to get into the action. Every night we would head up to where the big mall / bars / restaurants were. I can't even really find it on google maps, so fuck if I know where we actually were. The night life was crazy, bars are typically $20-30 cover but all you can drink once inside. Almost every night we ate at the same Mexican restaurant that was kinda on an alley-way that had a ton of foot traffic passing by. Awesome place to people watch.
Cancun was fun, but expect a lot of college age kids. Late summer is considered the "Mexican" travel season where the area is flooded with visitors from other parts of Mexico. As I get older and not into the stay up all night drinking and dancing scene, Cabo San Lucas was a much more enjoyable experience. More low-key, older more mature people, a bit more fun stuff to do (downtown area, seeing the Arch, went para-sailing, snorkeling, etc).
Bit vague, but hope this helps some.
