I went to CR for a wedding in Tamarindo, and obviously I had to explore as many beaches as possible! This time I rented a car with two friends, so that was easy. Costa Rica is very touristy and pretty expensive in general, it's also full of Americans and you don't even need to change $: they accept US $ and give you the change in colones.
Playa Coco. Lively town, a lot of stores and bars, sometimes loud along the main street, which is perpendicular to ocean. The beach is wide with dark sand, water not great, but there is a nice new waterfront where locals and tourists meets at sunset. I stayed at Hotel M&M, one of the few on the waterfront, rooms w private bath $20, recommended. I enjoyed few meals at the 3rd restaurant to the right when you get to the waterfront.
Playa Brasilito n Flamingo: nice wide beaches with some resorts, not much of a town.
Tamarindo. a nightmare: absolutely overdeveloped, overpriced, overcrowded, with tons of traffic and construction sites.The beach is nice and wide, full of surfers, surfers wannabe and wannateach. I stayed at Hotel Boteya de Leche, on the hill behind the beach, maybe 10 min walk from ocean. Familiar feel, friendly staff, wide kitchen and common areas, $15 for a dorm w/ private bath, $40 for a private room. No restaurants to recommend:expensive, and not  impressive.
Nosara, big surf destination. The original town is inland, the American town is sparsely spread behind the two beaches: playa pelada, intimate and picturesque, and playa Guiones, very wide but with no shade at all. Good waves though. I stayed in Guiones at Kaya Sol, nicely landscaped, with restaurant. $15 dorm, $68 for a cabina that sleeps 5. Good Asian restaurant next door.
Samara, my favorite town: wide long beach with shady palms, and several nice beach bars, activities and restaurants, e.g. Gusto beach, chic and tasty, and Tutti frutti, more rustic, both Italian owned. I stayed at Ahora Si, clean private rooms for $40, it is also has an amazing vegetarian restaurant. Italian owned as well... It's almost a colony! Great vegan food also at Luv Burger. Playa Corrillo next door is very nice: wide, with palm trees and no development at all, 5 min by car from Samara.

At 7am I'm at the bakery downstairs for a cappuccino and a roll. I enjoyed the roll even if it was a little heavy... It stayed in my stomach for the all day and I think it poisoned me! Nausea in the taxi to terminal caribe, nausea in the 4.5hrs bus to Puerto Viejo. When we arrive I can barely walk but an Italian connection saved the day one more time: Paolo, the owner of Cabanas Guarana' even if he didn't have room for us, gave Robbert a bike to go and look for lodging and he served me a ginger tea for my stomach. I love Italian expats! Robbert came back after almost an hour, it was hard but he found a great room w bath n wifi at Cabinas Talamanca. $20, excellent price for this town. I basically spent the rest of the day in bed agonizing. Maybe for the first time in my life I had no appetite at all!