Stay on a caye, go to the mainland for a day or two to look at ruins, and hike, etc. spend a few days snorkeling in the best snorkeling on earth. Go fishing.

I've been a couple times and had no problem filling a full week with fun there.

But I also think donner's idea is solid.

Another excellent one-week trip is Costa Rica. Man, that place is amazing, and close. We did two weeks there, and could have done more, but 7-8 days would absolutely be enough to have a blast.