Playa Hermosa is a great surfing beach that is only a few kilometers away from the hustle and bustle of Jaco Beach (the closest and busiest beach town to San Jose). Playa Hermosa is a laidback surfers beach. You can take surfing lessons or just hang out drinking ice cold Imperials or other tasty beverage from the shade, while watching some of Costa Rica's best surfers challenge the waves that break close to shore. At night you can relax or take a $5 taxi ride to Jaco for a night of debauchery.
If you are in or around San Jose Airport, I suggest that you take a Sansa flight (next door to the International terminal) to the following locations, which will get you near to most of the best beaches in Costa Rica in under an hour:
Barra del Colorado
Buses or rental cars can also be used to travel to the beaches, but driving is difficult and takes forever due to the poor condition of the roads and the crazy Costa Rican drivers.
The best beaches is very subjective and that will depend upon your particular definition. If you are into surfing then Nosara, Tamarindo or Punta Islita are your best bets for catching good waves any time of year in Costa Rica, because the Nicoya Peninsula has the best exposure to Pacific Ocean swells.
If you want untouched pristine tropical beaches then head to Puerto Jimenez or Drake's Bay and hike into the beaches of the of the Parque Corcovado on Costa Rica's Osa Peninsula.
If you want see sea turtles nesting at night then Tortuguero is your best beach for you. If you want white sand tropical beaches with a Caribbean flavor and crystal blue waters, fly into Limon and head south to Puerto Viejo or Cahuita.
My personal favorites are Playa Guiones, Playa Avellanas, Marbella, Playa Naranjo, Playa Hermosa, Playa Dominical and Pavones. But I'm a surfer and the reason I go to Costa Rica is for surfing. Here is a good site for Costa Rica Surfing.
If you want good food at the beach, head up to Tamarindo. I have met some great chefs in town that love what they do and it shows. Tamarindo has tons of places to stay and is generally considered a party town. This is the best place to get surf lessons in Cost Rica, hands down. There are plenty of things to do in Tamarindo that is why it has become a major tourist destination in Costa Rica.