No need to book anything in advance, Get out of San Jose, no need for more than 1 night, head to Arenal.
Why so many beaches and hardly any National Parks?
Keep in mind you can take Interbus between the popular venues, but it also usually needs a minimum of 4 people. The shuttle is private, not public. Public is the local bus. Shuttles pick from your hotel/lodging usually, where as the bus, you go to the bus station or stand on side of road.
There is nothing connecting Nosora/Samara to Montezuma, not even a shuttle. You would need to take many buses, and ferries, to get between them, figure 8 hours. Its 4 hours to drive between them, though in rainy season not easy either.
I don't know why you have chose Manual Antonio, it is a very small park, very touristy, very developed and hardly worth all that effort to get to from Pacific beaches, or Arenal area...
I would suggest you drop MA, and Montezuma, and pick one beach. Use Arenal as a base to do day trips or side trips to Tenerio/Celeste, and even Cano Negro. Then head to beach. Tamarindo has more shuttles/buses and services due to being close to Liberia, which has a International Airport and is a hub for transport. Its also a tad more exciting surf town.
Nosora is a sleepy town inland 2KM from the beach. Nothing is on the beach to speak of as they have enforced the maritime laws for years, so everything is set back from the high tide line. Its amino a surf area, not easy swimming.
Samara is a nice small compact town, easy to swim and beginner surf.
Tamarindo is best of both worlds, nice swimming, surfing for all levels, and fun town for nightlife/lodging options.
All the beach towns on this coast above, are 6 hours ride back to SJ...
http://nicoyapeninsula.com/nicoyasouth.php
I would also consider staying northwest to avoid heavier rains, Tamarindo area. Have you considered the caribe side? Its not only a lot more nature, wildlife and bio diversity, its easier to use public transport and get between points far easier and cheaper, and you have nice beaches and cheaper lodging, and more rustic and less developed than the Pacific side...
Consider going to Arenal, Tortuguero, Cahuita, Pt Viejo/Manzanillo-Ganduca and raft the Rio Pacuare back to SJ...