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.