Hello Megan,
I currently am living in the QUepos/ Manuel Antonio area. Bicycles are a way of life here, especially in Quepos itself and its surrounding area. The reason is that the town and region are flat, so it's rather easy to get around by bike. I would not try all the things the locals tend to do on bike, they tend to wing it/risk it, but the driving around here is an adventure in itself. Manuel Antonio is a different story. It's a hill, and a very winding, at times very steep, hill. There are no sidewalks, in fact, there is just the road which is rather narrow for the traffic and activity it endures, and I can assure you it is not safe to bike this hill or road. The cars fly by, the local buses are huge...they FLY by as well, there are many blind spots, scooters, motorbikes, ATVs, occasional walkers/runners, sometimes a horse or two, stray dogs, chickens, fallen trees and God-knows what else,that occupy this hill and its road. It's a fun ride...in a taxi or a car or the local bus. That said...I hike the hill daily because I know every curve and I still probably risk my life:) And yes, the buses come THAT close to you.
Bikes here are NOT cheap, especially if you get them from a local bike shop. I priced a Trek and it was well over $450 for something that was decent, NOT great. There are more options for shops in San Jose, and maybe some local chain stores like Gollo will have bikes under $300, or even under $200. I would maybe consider renting a scooter but then, is it safe. Taxis are fairly cheap here. The local bus is frequent, reliable and costs about 45 cents. It will take you from center of QUepos all the way to the beach and National park.
There IS a gym in Quepos, right before the first hill that the Manuel Antonio main road starts. It is called Gimnasio Mucho Musculo, it is air-conditioned (trust me, IMPORTANT FACTOR), they have beverages and shakes for sale, a class schedule, spinning bikes and a good collection of workout and weight training gear, mats, free weights, core balls, etc. I am a gym rat and have been a member of every gym in NY, so finding this here was crucial. This gym has lots of charm but please understand, it is not a big-city, glossy, shiny, glam gym. It has what you need, the owner is American, they speak English and have long hours to accommodate you. I pay monthly and it costs me $24 roughly. They do offer daily/weekly rates as well.
Hope this helps! Here is a link to their Facebook page for more info:
facebook.com/?ref=home#!/pages/Manuel-Antonio-Costa-Rica/Gimnasio-Mucho-Musculo-Gym/264935700013?ref=ts