A couple things for first timers. Manuel Antonio sits at the end of the main road coming from Quepos. There are relatively no side roads. The bus runs every 30 minutes and will take you right to the park (it's virtually impossible to get on the wrong bus). Learn the buses!
The park costs around $7.00 (under 12 free). Near the park, you will be confronted by people wanting to be guides. We were offered prices (for 4 people) from $50 to $160. I'm sure we could have found one cheaper, but we weren't interested. You absolutely do not need a guide, and the park is too small and too well marked to get lost. That said, a guide will almost guarantee that you will see snakes, monkeys, sloths etc., as they keep in touch with each other and let each other know the animals' locations. Some also supply food, and have very powerful telescopes that you can take pictures through.
The choice is yours, but be sure and get a good rate.
We went during the rainy season and had most of the trails to ourselves. The main beach was the most crowded (20 people) and is where we saw the most wildlife (monkeys, raccoons, and a large German couple changing out of their swimsuits in front of everyone). Watch your possessions! There was a raccoon going from bag to bag seeing what kind of goodies he could steal. This baby was not shy.
The park is beautiful, with well marked trails and gorgeous beaches. There is no sense of roughing it. At times they have a concessions stand open, there are bathrooms, primitive showers if you go swimming, and even pay phones. It's still beautiful though. None of it has pavement, or sidewalks.
It's mostly like going to a large nature center, but an extremely beautiful one with wonderful beaches.
If you take the back way out down the service road, you will exit by some small hotels that are tucked away in the woods. There are some small stands here selling food and drinks. As you exit, make a left to get back to the main road.

