Depends on your budget, but I'd recommend either Sharm or the Maldives.
Sharm has more to do in terms of trips/shops/bars, etc outside the hotel. Maldives has better diving and snorkelling - generally a hotel will take up an entire island (which you can walk around in a few minutes) and has a fringing lagoon and reef. Often the reef has had a few channels cut through it, so you can just swim from the beach to the outside of the reef, where the number and variety of sealife is amazing. However, there is not as much to do as in Sharm - a few boat trips from the island, some entertainment at night and other than that, just relaxing. Diving is excellent, usually with the choice of a couple of boat trips per day or shore diving from the island.