Well, if you go to Progresso then you can go to Chitzen Itza which is a huge and fabulous main Maya site. Xumal or however it is spelled is also close to Progresso. In Belize you have choice of Xunatunitch ot Altun Ha and I highly recommend the former. There are small minor ruins in Cozumal called San Gervesio.
Tulum is great more of a city them a regilious site which I liked for more of a feel for life..the view is awsome..have been to Chitzen Itza also very impressive.. San Gervesio is nice for a day trip on Comezel still gives you time fora quick trip through town and some shopping and a few drinks.
Just back from Valor. Went to Altum Ha and River Wallace. It was very nice. We went down the river from about an hour. Got to see alligators, manitees, dolfins, monkeys. Then a stop for lunch, bathrooms and a beer. There was some shopping there. On the bus from there for another 15 minutes, Ruins were small but tour guise very good. Enjoyed this tour. I took this tour through the ship and am glad I did as we were late getting back. Friends took the Tulum. Said they were beatiful with the ocean backdrop but the trip was long.
Chitzen Itza gets my vote. Only drawback is the amount of bus riding to and from (we did it as a company tour out of Cancun). The place and temple are huge, and there are plenty of other interesting sites there. Ive done a small set of ruins by horseback in Cozumel. Didnt compare to CI. My dad did Tulum and from the pictures he took, it still didnt compare to CI!