People usually either love or hate Tamarindo, often referred to as "Tamagringo." If you prefer to stay in developed area that has many of the amenities of home (such as Subway and Pizza Hut) and a nice beach to boot, Tamarindo is a good bet. Once a sleepy town, it's become a popular spot for North American and European ex-pats, many of whom have purchased property here. There's a lively party scene, good shopping, an abundance of accommodation, and some lovely beaches a short car drive away. Just don't expect to learn much Spanish.
Mal Pais and the neighboring town of Santa Teresa are located in the southwestern corner of the lovely Nicoyan peninsula and have been a surfing destination since the 1970's. Once a remote and undeveloped spot consisting of dirt roads, Mal Pais and Santa Teresa have officially been discovered, for better or worse. However, its chilled out, surf's up spirit remains (as does its dirt roads), and many people who come for a few days end up staying longer.
If you prefer idyllic, white sand beaches over pounding surf, head to Costa Rica's Caribbean coast. Approximately 1/3 of the people have Afro-Caribbean roots, giving this area a very different feel than the rest of the country. The locals speak an English patois, the music is calypso and reggae, and the vibe is as laid back as you can get. Two of the most popular towns to stay in are Puerto Limon and Puerto Viejo.
This is also the wettest part of Costa Rica, so if you tire of the beach, you can head inland to explore the incredibly lush jungle.
For most visitors, a trip to one of Costa Rica's cloud forests is a must see. The Monteverde Cloud Forest is the most well known, and hence the most crowded, receiving about 200,000 visitors each year. If you prefer to see more nature and less people, head to the nearby Reserva Santa Elena, which is just as beautiful and only receives about 20,000 visitors per year (though this number is increasing). Supporting one of the first conservation projects in the country managed by the local community, your entry fee to the park will also be benefiting a good cause.
This park, shared by both Costa Rica and neighboring Panama, is a UNESCO World Heritage Site. The Costa Rican portion of the park encompasses approximately 750 square miles, making it the largest natural reserve in the country. Accessible only by road, its remote location and rugged terrain makes this park is one of Costa Rica's best kept and least explored secrets. The easiest base from which to explore the park is from the village of Altamira.