It's an easy flight down there, we stayed at Frenchmans Reef and it was ~$20 from airport. There's a supermarket there where many stop and load up on groceries. There's a small open air restaurant/bar right next to the duty free mall that has an amazing happy hour--excellent food for dirt cheap.
We went in 2008 and also stayed at the Marriott (we chose not all-inclusive). We should have rented a car because it was expensive taxi rides (pay per person, not per taxi) to the other parts of the island (I don't want to pay $20 every time I wanted to leave the hotel). Remember that they drive on the left. We went in September and there were things already closed for the season and some things were only open on certain days (probably when cruise ships were in port), so plan ahead if there's something you want to see.
Some of the hotels are built on the sides of hills, so there might be a ton of steps to get to the beach.