I stopped in Belize when it was a relatively new stop for cruise ships (about 2 years ago). I say that because I don't know if they have added more touristy stuff, but my brother and I weren't that thrilled. We didn't have any problems walking around, but we did stick out like a sore thumb.
I agree with some other posters that there wasn't a lot of things to do right off of the ship. But, since my brother and I aren't the tour-type people, we set off for a walk into town. There wasn't anything to see (and we searched).
However, on the way back to the ship we found the nicest gallery just outside of the terminal. As you leave the terminal it is on your left. A nice white building. Anyway, it is owned and operated by this lovely woman who moved from Atlanta to Belize with her husband and kids a few years ago. The shop represents many local artists and craftmen. She has some lovely things at reasonable prices, including art work, glassware and purses. We wound up spending a couple of hours browsing, spending and speaking with the owner. It was the only part of Belize that was wonderful. If the shop is still there, make sure you stop by.