We (ten of us) stayed at Banyan Bay March 7-14, you are going to love it!! Seven of us are divers and we had booked our diving with the dive shop at Banyan Bay (Bottom Time Divers-GREAT-would use again in a heartbeat) before arriving. However, 2 of the days the current was too strong and the dive boats did not go out. They made the $$$ adjustments for us. Some of the trips you mentioned, such as Lamanai and Blue Hole, are weather dependant. We did not book these until we arrived and had no problem. Three of the women with us do not dive, but do snorkel. They would just go down to the pier at Banyan Bay the night before they wanted to snorkel and sign up, boats go in the morning at 9AM or in the afternoon at 2PM. It was great and Bottom Time was very accommodating! From everything we encountered, it was the same price if we went through the dive shop before arriving or just on the spot. Lamanai prices are pretty standard....$125!
Have a great time!! I'm jealous!! Pamela