we spent the day at manuel antonio. went in through the new entrance to the park. on our way to the beach we saw a monkey and a sloth. later on in the day we also saw deer, racoons, and howler monkeys. It was definitely well worth the 20 minute walk to the main beach and another 15 minutes to the twins beach (playa gemelas). Do not forget to bring your lunch or walk back out to entrance and there is a great filapino restaurant Parque sushi. Excellent.
Just remember on the beach to protect your belongings from monkeys and racoons.
also it is now rainy season so be prepared for afternoon showers. you do not need a guide since just watch where everone else stops.