I have been there. I spent a week in San Pedro, on Ambergris Key. It's very low-key there. I did not spend any time in Belize City, and I don't think there is a strong reason to do so. San Pedro is about a 20-minute flight from Belize City, and if you are lucky, you will fly in the seat next to the pilot (it's a really small plane).

The main attractions in Belize are the water and the Mayan ruins. Belize has the second-largest barrier reef in the world, and the diving and snorkeling are fantastic. But because of the reef, there is not good swimming off of San Pedro - the water is very calm, but overgrown with weeds. You can take a boat out to the reef for snorkeling for around $20. You can also take a boat trip into the jungle to see Mayan ruins - this is a worthwhile day trip.

English is widely spoken, and the US dollar is widely accepted with a fixed exchange rate. And when you leave, be sure to stop in Eddie's Bar in the departure terminal...