One of the most popular things to do while in Manuel Antonio is to take a tour. There are a variety of rain forest canopy tours and Zip lines, white water rafting, horseback riding, mangrove tours, sunset sailing, and other tours.
Typically tour operators will pick you up at your hotel and take you to the tour. Often a meal is provided with your tour, but ask in advance.
If fishing is your scene, there are a variety of companies that rent boats, gear, and and whatever you need to make a fishing expedition successful.
You can also scuba dive from companies based in Quepos.
Manuel Antonio National Park was established in 1972. It is the smallest Costa Rican national park, but one of its most beautiful. It is widely visited by international tourists and open Tuesday to Sunday. There is a fee for its entry, one rate for international tourists and a local rate. It is well known for its beautiful beaches, wild life and hiking trails. Forbes magazine listed it as among the world's 12 most beautiful national parks and Tripadvisor has placed it in the top 20 most beautiful beaches in the world to visit. Tour guides are always available. It is a highly recommended place to visit while in Manuel Antonio. The entry is at the end of the road away from the main beach and restaurants.
Within 60km (about 45 minutes drive in either direction from Quepos are a variety of places to see. Toward San Jose you will find surfing beaches and a variety of small villages and towns. The largest, Jaco, is close by. It is bigger than Manuel Antonio with a surfing beach and a large amount of restaurants and bars.
If seclusion is what you want, then travel further south where you will find secluded beaches in Matapalo, Baru National Reserve, Dominical, Uvita, and Ballena National Park. In many of these areas you can walk on beaches for miles and be virtually the only person.
A great day trip is to drive to Nauyaca Falls - a 30 minute drive to Dominical then drive inland for about 10 km on good highways. You will then have a 5 km nature hike on a popular trail into these fantastic waterfalls.
The highways towards Jaco or going south are excellent - no need for a 4 wheel drive.