I was there in Dec and stayed at the Iberostar. There wasn't much reason to leave the resort if you just want to hang out at the beach or pool. There are vendors in the lobby's of most hotels that sell excursions. There was one that was a day long that took you to see some ruins and give you a historical tour of the area. We didn't do this.
We did a dolphin swim which was decent. Some of the snorkeling excursions looked fun, but my wife wasn't too crazy about it. There's one that is takes you to a floating island where you eat drink, kayak, snorkel, etc all day. The only problem with that is there's no shade and you'll probably roast.
If you booked on a site like Expedia or Orbitz I would book transportation through them ahead of time. It's pretty hectic once you get out of baggage claim.
The local street vendors can be pretty pushy and will try to sell you everything. If you really want something to take home they can be haggled a lot lower. Everyone will call you "friend" and try to strike up a conversation about where you're from before trying to force you into their store.
The food is different, mostly because there are a lot of Euro tourists so it's geared towards them. If you're staying at an all-inclusive, the first thing you should do is go to the reservations desk and make as many dinner reservations at the resort restaurants as possible.
Any other questions just ask.