Within 60km 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 larger than Manuel Antonio with a surfing beach and a large amount of restaurants and bars.
If seclusion is what you want, then in the other direction toward Panama, secluded beaches await 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.
One of the most popular things to do while in Manuel Antonio is to take a tour. There are a variety of rainforest 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 arguably 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, wildlfie, 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 10 most beautiful beaches to visit. Tour guides are always available. It is a highly recommended place to visit whil ein Manuel Antonio. The entry is at the end of the road away from the main beach and restaurants.