Just wanted to report that while in MA a few weeks ago (mid March), my group of 4 took a guided tour of the park with Juan, as well as a kayak tour of the Damas Island mangroves, and had a great time.
Juan is a great guide and a really nice guy. He did all his guidely duties perfectly, showing us many animals and plants, and made it really fun. He was also probably the main reason we didn't wind up red and itchy due to the Manzanillo trees.
The kayak tour was also very fun, and definitely a good workout. We saw a lot of interesting water birds, and even got to see some Capuchins toss a baby racoon out of a tree (I think he was OK, though, poor little guy). The soda Juan took us to for dinner afterwards in Quepos (La Cubanita) was also great.
Bottom line, I would definitely reccommend Juan and his company Aguila tours to anyone looking for a guide or activities around Manuel Antonio.