All inclusive is the way to go if you're planning on just staying at the resort and not going off to too many offsite bars, clubs, attractions, etc... Cancun has a ton as you can see from above. Personally, I'd recommend heading south 30-45 minutes into the Riviera Maya and getting away from Cancun. It's less crowded and more relaxing. There's too much to do in Cancun, you'll never stay at the resort, and you'll end up burning through a lot of money in town. You're better off just going with a non-inclusive motel room if you're staying there.
Do your research and pick the resort that is right for you in terms of the rooms, dining, bars, pools, and activities.
If you don't have kids, I'd absolutely go to an adults only resort. I went to El Dorado Royale about 5 years ago, paid $2800 for 2 adults for 6 days.