Hi DeLane, my name is Ignacio and I am a tour guide here in Belize. Concerning the mayan sites of Tulum and Xunantunich, I would say that i have heard from many people (tourist) that it is not so much what you see but what you learn. Our guides here are well trained and knowledgeable and you will learn a lot more about the mayans here in Belize than you would in Mexico. Now you would say that I am a bit prejudice about our guides here but this comes from tourist like yourself that has express this.
We have several sites that you can visit when you get here. Lamanai is one ot the popular tour because it has a boat ride of about 1 1/2hrs on the New Belize River before you get to the maya site of Lamanai. On this ride you get to see crocodiles, iguanas and birds. Then you have Altun Ha maya site which is also very popular with cruise ship passengers because it is very close to Belize City (approx. 31miles) and it has a beautiful history , plus you get to see 13 structures and climb a couple of them if you want. But why cruise ship passengers like this tour is because they get back to the city with plenty of time to shop around for souvenirs and stuff.
Well I hope that this information was helpfull to you. If you need further info or assistance please feel free to contact me at firstname.lastname@example.org
The problem with the Tulum excursion is, if you leave from Playa Del carmen, that you spend far more time on the bus than you do at the ruins. We spent over 2 hours each way on bus, with about an hour and 15 min. at the ruins. While it was beautiful there (and very hot) it just wasn't worth the aggravation. If you do go, wear swim wear and take a dip in the sea below the ruins. That is truly an awesome sight.