After having gone on guided walks with naturalists at Mistico hanging bridges in the Arenal area AND in Monteverde and not having seen much in the way of animals, I was hesitant to book another nature walk. However, knowing that Manuel Antonio Park is famous for its wildlife, I booked with Edwin and IT WAS AMAZING!!
Our guide, Saul (pronounced Saw-OOL, not Sol) Guzman was absolutely fantastic. He was hustling to find wildlife, and also talked with other guides to give/receive information. His ability to see things with his naked eye and then immediately set up his scope so that we could see what he was seeing was excellent. We saw three sloths, howler monkeys, capuchins, two deer, a (dead) gladiator frog, colorful red/blue crabs, "Halloween" crabs, a grasshopper as long as my hand with red wings, the "Jesus Chris" basilisk lizard, a rare stick bird, several agouti, a white-nosed coatimundi lazing about in a tree, tiny green, black, and red grashoppers, hummingbirds, and many other things that I can't remember. I highly recommend Saul and this tour!
Having a private guide was also great because the four of us could look in the scope and not have to wait for others - or worse, wait for others to take pictures in the scope! We went at 8am and there are ALOT of people going at that time - Saul said much fewer people go on tours at 10am, so that is another idea if you don't feel like going at 7am (which was too early for us).