We took a fabulous all-day excursion two years ago... In the morning, we drifted up the Old Belize River in a small boat (about 15 people)and saw crocodiles, howler monkeys, beautiful colorful birds, etc. Stopped for lunch on the banks at a quaint restaurant serving native food (great). Then we boarded buses (vans, actually) for a quick trip into the interior for a tour of Mayan ruins at Altun Ha. We were able to climb ancient pyramids and wander through the ruins of the royal palace. Lots of fun. I would think it would appeal to a 12 year old.
It was called the Old Belize River/Altun Ha excursion... don't know if it's still offered.
Last year we went on a different cruise line to Belize. We took the Lamaini excursion which included a boat ride up the New River to the Mayan ruins. It was fascinating--especially the birds and unique flora and fauna we saw. Our kids loved it, and a monkey tried to jump in our boat! Our tablemates did a swim with the rays excursion off a catamaran, and had a ball. The sea was a bit rough that day so I'm glad we didn't do that with the kids, but it sounded great. Some of the swim with the dolphins or swim with the stingray excursions are just a waist-deep enclosure that you walk around in with the fish, and not very exciting. Some involve actually swimming with them in the ocean, so just be sure you know which one you want to do. Have fun!