Well, I definately will say you made a great choice regarding Banana Beach.
I've never been to Hawaii but I would also agree with the snorkling at Hol Chan to be superb....sign up at Monkey Business at the hotel to go with Alfonso...He is great..
You can definately do the cave tubing in one day but it is an all day trip. The trip with Serious includes the Belize Zoo and lunch at Jaguar Paw Resort. It's a long boat ride to the mainland but definately a worthwhile trip.
I would also say to either sign up for the Bacalar Chico trip or the Rocky Point Picnic stop. Rocky Point trip you do 3 snorkel stops and literally catch your lunch. Actually on our trip we would have starved had it not been for our guides spearing the fish but it was fun trying.
Other than that I would agree....Play it by ear.....