I left out towns and villages when I was thinking of things to do, but I find them interesting, each in its own way.
Belize City - by far the biggest and the baddest town but still has some interesting historical things to see. It was the country's capital until Hurricane Hattie devastated it in 1961 back when the country was known as British Honduras. There's the Governors House, the Swinging Bridge, Baron von Bliss Memorial, the old American Embassy, St Johns Cathedral, Belize Museum (former BH prison), the waterfront and Tourism Village. Best not to travel alone. Stick to the main streets. Try to blend in. Never go anywhere after dark.
Orange Walk - feels like Mexico. Close to Lamanai.
Belmopan - now the country's capital, moved far enough inland that it should survive the next big hurricane. Some interesting government buildings. The American Embassy looks like a fortress. Not much else there except good Mexican food and a nice park on the river.
Spanish Lookout - turn off the Western Highway and within five minutes you'll think you're in Ohio. The Mennonites are an industrious lot. It's the go to place for everything from farm equipment to pre-built houses, granite counter tops and windows. If you need something built, just ask a Mennonite from Spanish Lookout.
San Ignacio - rockin' little town and fairly well spruced up for tourists in the last few years. Great place to buy marijuana if one is so inclined as it's usually offered to me within minutes of walking around. I always pass as I don't want to take a chance on spending my vacation in the Hattieville Prison awaiting extradition back to the US. SI is in the Cayo District and is a jumping off point for jungle adventures.
Dangriga - largest Garifuna village in Belize. Very poor but also very bustling with activity. Sitting on a billion dollar view of the ocean.
Hopkins - a smaller version of Dangriga just to the south. Growing fast with ex-pats.
Placencia - a small fishing village meets tourism. Even though it's on the mainland, it has a definite island vibe because the peninsula is barely wider than the highway in some places and even in the wider spots you are almost always looking at water.
Punta Gorda - in the Toledo District, which is the very most southern part of Belize and that probably has something to do with it being the poorest district as well. Probably the least touched by tourism. Right on the Bay of Honduras but otherwise surrounded by jungle and lots of very small Mayan villages and ruins. This is my personal favorite area to visit and I always make it a point to spend at least a few days there, usually in the middle of a dive trip to Placencia.