Your best bet is take the local bus (50 cents) to the public (free) Manuel Antonio beaches, or you can pay $10 each and go inside the park (park is closed on Mondays) where the beaches are even more amazing. If you go in the park, you need to pack food and water, as there are no services within the park. If you are outside at the public beach it is perfectly safe and you can rent chairs, umbrellas, boogie boards, surf boards, etc and waiters will come around and offer to get you stuff from local restaurants, etc or you can just walk to one of the many options around that beach area. Both are excellent options and only minutes on the bus or by taxi (around $6-$8 each way). There are also taxis (red) that wait around the public beach area to take you back to Quepos, or any restaurant is happy to call you one. Once you see it, you will see it is super easy! Enjoy!