Fish the flats for Tarpon, Barracuda, Bone Fish, and Permit. Don't underestimate the Permit fight. Bone and Barracuda are almost translucent on the white sand flats. Son and I did this together. GoFishBelize.
Take a boat tour to pick up some Conchs, do a little spear fishing, take the catch to a private island for a Belize BBQ. Wife and I did this with two other couples and all our kids who have played ball together since they were little (teenagers now).
Take a catamaran tour out to Hol Chan (snorkeling, I know you are a diver), then proceed to Caye Caulker for eats and drinks, returning to San Pedro/Ambergis. Did this on the same trip with the larger group of freinds totaling about 20, so we had the boat to ourselves, which made it better.
We were there March 2020 on Covid US Meltdown week. Most everyone had cancelled except for a very few Americans and Canadians. Very few. Places still open, but had our pick of tables in restaurants, eateries/bars, and tours. Even better that one of the couples do this every spring break so they casually behind the scenes set a lot up and knew the right contacts for the activities and places to hang, drink/eat.
Watching the US meltdown from there was surreal. Getting of Ambergis/San Pedro to the mainland at the end of meltdown week was really weird. We were leaving as a group of families 4,4,3. Well, our fam of 3 got bumped at the last minute on the "puddle jumper", because of "routing". When we met up on the mainland, and asked the others a few questions, it became clear that our 3 seats got "filled" with someone wanting off ($$$) more than us cause the mainland flights to the US had no more seats and our seats were filled, and it was almost becoming a panic to get back, for some folks. I got stopped at the carry on Xray on the mainland airport because I thought I had lost my knife back on the island and it had fallen into my backpack. I emptied it prior to packing the checked bags and could not find it. The Xray did. With the sign behind the guy that pulled it out, clearly saying "detainment" for anyone with items like that, I figured I was screwed. I think they were ready for the tourists to go at that point, cause he looked a the sign, back at me and waived me on, dropping the knife in a slot in a bucket that looked filled with like contraband.
If you like cigars, take your own, or buy only from Havana Cigars in San Pedro. Everything I saw was either fake or kept poorly.
Enjoy!