Manuel Antonio is a very accessible park. There is a plethora of hotels, cabins, and rental apartments on or within walking distance of the beach and park and bus rides on the half-hour from Quepos. Home to some easy and picturesque trails, you are guaranteed a shot of a sloth or white-faced monkey within the park and a story of how a lizard stole your sandwich. Be sure to take the offshoot trails (all over the place) that often lead to lovely beach coves.
Quick Tips:
Manuel Antonio is a great park for people with kids or who want to be sure to see some wildlife up close. Personally, I was not very impressed. The monkeys are similar to the chipmunks of Yellowstone (overfed by tourists and not to be trusted near an unprotected picnic table). There is often a zoo of tourists and the beach can be crowded. I spent nearly 6 months in Costa Rica in a small town near Quepos, so I was here often. Walk a ways down the coast and you'll find some private shade, eventually.
If you are looking for a less-travelled region or animals that don't mug for the camera, take a rain-forest hike in the Osa Peninsula. And if it's a beach, you need take a short drive (or bus ride) out of Quepos to any of the surrounding coastal towns. Most have access to the beach, and it's much more private.
It's good to know some Spanish, because many of the bus drivers or people you would ask for advice will not know English.Best Way To Get Around:
I always rode the bus. From Quepos to Manuel Antonio it cost me about 100 colones, roughly .25 cents. The exchange rate has no doubt changed in 2 years, so check that out. You pay on the bus, so don't go to the ticket counter--that's only for bus rides to farther-out cities.
There are also taxis. The legal taxis are always red car/vans of various makes, models, and shades of red. The pirate taxis are not always to be feared and are usually cheaper. They can be found behind the bus station in Quepos. However, ride at your own risk, because the police often set up traffic checks to crack down on these illegal transporters.
If you plan to head to one of the smaller towns, just make sure you know the bus schedule. They can at times be few and far between, but it's worth it to get out of Quepos for a while.