If you have a cardiovascular condition or physical disability - be careful.
We had to step through canopy crossbars into the riverboat, which proved difficult for several people.
At Lamanai, it's a rainforest, so it WILL rain. Take a goretex jacket, and if asthma, an inhaler, since the very high humidity paired with the walking may make breathing more difficult. I wore high traction volleyball sneakers, which helped on the at times pretty slippery steps and paths.
If it rains, you can only go to the second level of the Big Temple, not all the way to the top, for safety reasons. Since there are a lot of groups, you don't get pushed, but you don't idle by the sites. The guides explain the background/history.
More interesting than Lamanai to me was seeing every-day Belizan countryside.
I've lived in 3rd world countries, so I don't go sightseeing and expect Diamonds International. Very interesting cultural mix in Belize.
Read SteamboatWillie118's full Oosterdam cruise review
Holland America Line