My husband and I are traveling with 3 teens to CR 8/2-8/12. We fly into SJO and will likely follow your suggestions to visit Arenal area for first 2 days. Looking to then head to a beach town as base camp for remainder of stay. Trying to decide between MA and Montezuma. We are looking for a vacation rental and want to stay as close as possible to walking distance to nice beach, shops and restaurants. We will likely have a car but want kids to be able to tool around on their own from our beach spot. Don’t mind a hike, shuttle or quad to beach. Love idea of staying in jungle like area close to beach too.
Between Manuel Antonio and Montezuma, you might have a better chance finding a vacation rental near the beach in Montezuma. Most of Manuel Antonio’s vacation rentals are located up on the hill, away from the beach, so would require a walk and bus ride. Montezuma has some closer to the beach and isn’t as developed. Most of Montezuma’s restaurants and shops are also right near the beach, whereas in Manuel Antonio they are spread out (some at the beach and others up on the hill). You might also check out the town of Samara, which would be an easy drive from La Fortuna and has a lot going on right at the beach. There are some vacation rentals available there too.
Best of luck and have a great trip!