We went to Altun Ha 2 yrs ago and it was wonderful. The particular excursion we went on was a tour of Altan Ha (the guide was very entertaining) and then an 'authentic' Belizean lunch and a river boat ride. I have a relatively short attention span but I was not bored during any part of this.
The guide for the ruins pointed out various things that I wouldn't have noticed, explained the significance of many carvings, structures, etc. but it wasn't like sitting in a classroom. The river boat ride was unique -- there was a young boy on the front of the boat and his job was 'spotting'. He looked for (and found) crocodiles, black monkeys, birds, etc. The driver of the boat would then slow down or stop and let you take pix and tell you a little about whatever it was the spotter found.
I'm sure which ever ruins you pick will be fun. Enjoy