we loved Lamanai - first there's a good drive through the country side - hopefully with a guide that will point out all the interesting stuff, then a long ride down the river - again lots to see both villages and wildlife, the a great lunch, then a hike to very extensive and interesting ruins then the reverse trip home.
We also did the tubing, skipped the zoo so we could be on the river all day. It was also great, but if you have to choose the ruins win by far IMO.
We will see Altun Ha when we return this Sept.