Maruba does day spa packages where people come from Ambergris for the day and do treatments. You can contact them via e-mail direct for a quote on what you want or you can arrange it when in San Pedro by contacting Alexandra Nicholson at 226-3770. She lives in San Pedro and is part of the family that owns Maruba. She also does treatments there on Ambergris.
If you go, keep in mind you are going to the jungle in the rain forest. There are all kinds of creatures living there and if you are not smart enough to take precautions, like using insect repellant, you might get a bite.
PS: we are also coming into the pica pica season, so keep that in mind when diving or snorkeling.