Relaxing view, isn't it?
If you get tired of lying on the beach and of the jungle trails, you may try snorkeling. The life under the water is as rich as the life on the land. You have to bring your own mask, because there is no place to rent it in the park.
Another recommended activity outside the Manuel Antonio Park is the sunset sailing tour. You can book these tours through your hotel or in the town of Quepos. If you are lucky, you may get to see whales and dolphins. Most boats stop for tourists to go swimming among local fish. The sail lasts around three hours. Note that some tourists not accustomed to the sea get seasick. If you like fishing, you may also join a fishing tour.