Cruise ship passengers come ashore at the Tourist Village in Belize City -- the Village is like a small shopping mall.
If you go outside the Village, local tour guides will be waiting somewhere nearby. They will have placards showing the tours they offer, so wander around and talk to a few before deciding. Also, check that they are licensed tour guides (a requirement), and check the condition of their vehicle if possible.
I can't say what's "best" for you, but some ideas are the Zoo, ziplining, airboat rides, tubing, snorkelling, museums, Mayan sites...