The Vaca bush tours are at Martz farm in Cayo -- good inexpensive place, though I don't know much about their horses.
A number of lodges in Belize offer horseback riding including Chaa Creek, duPlooy's, Crystal Paradise and Chan Chich.
Probably the two best-known horseback operations are Banana Bank Lodge near Belmopan, which has fairly recent built a large new stable and riding complex, and also Mountain Equestrian Trails at Mile 8 of the Pine Ridge Rd.
You can figure on paying in the range of US$65 per person for a full day horseback trip, and around US$35-$45 for a half-day.
These things are all easier arranged if you are already on the mainland. Not sure if anyone on Ambergris Caye sets up horseback riding trips but it probably can be arranged. Banana Bank might be the easiest since it's not so far away.
Altun Ha and Lamanai are the two Maya ruin tours that are done from Ambergris Caye. Of the two, I think Lamanai is the more interesting trip, and the more interesting site, plus you will see lots of birds, howler monkeys and other wildlife. You can also go by plane and visit Tikal.
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