Depends on whether you prefer fun & relaxation in the water, or a little history and hiking... You choose, but my personal favorite is the Manatee Watch/Snorkeling/Beach BBQ trip with SEArious Adventures. It's a full day, $75, and it includes a fabulous lunch, drinks (pop, water, rum punch, Belikin). You'll see manatees playing, have lots of opportunity to snorkel, and enjoy lunch on a quiet little island (quiet if there are no other tour companies there on the same day). Then off to another snorkel spot with sharks & rays you can swim with on the way to Caye Caulker, where you finish off your day with a stroll down the street, look in the shops or cool off with a drink at a bar. It's a great day. I've done it twice and will do it on every trip.
If you like more history, opt for SEArious' Lamanai trip. It's a long day with lots of boat travel, but you get to see a lot of wild life (crocs, birds, howler monkeys, etc.), and the Mayan ruins are amazing.
You can find SEArious on the dock at Ruby's. Anyone can tell you where it is once you get there.
Have a blast!