I would definitely do Ian Anderson's river cave trip, address is [email protected]
Ride horses at Banana Bank, [email protected]
It is a great place to stay too, you can hear the jungle sounds at night and they have wildlife on site, you stay in thatched roof cabanas. Visit a few Ruins- Xunantunich and go up into the Pine Ridge area to see Caracol, waterfalls, etc. Lamanai is a great ruins trip and different from the others.