I did Belize several winters ago.
The main attraction is the barrier reef; the second longest in the world. The beaches are not terrific, since the reef stops the sand from building up.
Belize city is a half hour-45 minute boat ride from the reef. It has a Hyatt.
Most people stay on the islands off the coast, which are closer to the reef. There are no brand name hotels on the islands. The islands are called "Cayes", which is pronounced like "keys". (Like the florida keys.)
There are two main islands, the rich one and the poor one.
The rich one is called Caye Ambergis. It has more people, is more expensive, more nightlife, more expensive tours, etc.
The poor one is called Caye Caulker. It has less people, cheaper, etc.
We stayed on Caye Caulker and totally roughed it, but had a mean good time. We got a room in a "hotel" for 12.50 a night, which had a shared bathroom in the yard (hard to describe, but the bathroom was a building with a door for each stall).
But, you can get nice rooms also in places, american style. Think 50 a night or so, I think.
Snorkeling trips to the reef cost about 50 bucks a day. Scuba diving is like 100 bucks, but you need training and whatever. Trips to the "Blue Hole
http://en.wikipedia.org/wiki/Great_Blue_Hole" are a bit more, but you should do it especially if you are scuba diving (we didn't, we were cheap).
At night, you should make a BBQ with some fish or stuff you bring, and drink the local rum-One Barrel Rum. It's delicious. There's karaoke and stuff, or just sit on the docks and watch the pelicans.
On the mainland, there is all sort of stuff. There is parrots, and jungle, and caves, and wildlife refuges, etc. We did that for a day.
The local language is English. The currency is Belize Dollar, with a fixed exchange rate of 2-1 (2 Belize=1 dollar).
You should buy fresh fish from the fishing people, and cook it. We did.