We just came back from a quick trip to Belize and Chetumal. There's a new hotel in Chetumal called Noor Hotel, which isn't listed in the hotel listings. Skip the Holiday Inn (where we spent the first night with a broken shower and a dripping air conditioner) and go straight to the Noor. Very sexy hotel in what I would call a California modern style right on the bay and a better rate No elevator and no decent breakfast but if you like style, fantastically soft and comfortable sheets, a big spacious rain-style shower, a balcony facing Chetumal Bay, WiFi, and a nice pool, then stay here. (Don't forget your mosquito repellent.)
And I highly highly highly recommend a day trip or even a night in Mahahual. Gorgeous warm ocean water, soft white sand, and amazing conch ceviche. Check out Cat's Meow for their pina coladas and cheesecakes.
And wherever you go, don't forget to pack half as many clothes and twice as much money as you think you'll need.