I am going to be in Costa Rica for the last week and a half of May. I'm trying to decide between heading to the Nicoya Peninsula or going to the Puerto Viejo area on the Caribbean coast. Beautiful beaches and warm water are a high priority to me. I want my time in Costa Rica to be mellowish (read: not Cancun spring break) but I also don't want to "escape" so far that the trip is desolate and boring.
Does anyone have any suggestions on which direction I should head, and specifically, which cities you think are amazing?

Thanks.

The water's much warner around Puerto Viejo, and the vibe a lot more laid-back.

Puerto Viejo is beautiful and is a cool place to party; lots of kids, drugs, vegan restaurants, but the beach isn't all that and the rainy season is about to hit (well its pouring so it already has)and Puerto Viejo gets all the weather systems. On the southern tip of the Nicoya on the Pacific side is a place called Santa Teresa/Mal Pais. The most beautiful beach in Costa Rica and not yet built up like the ones closer to San Jose. The northern Pacific coast is alot drier than the Caribbean which is a big deal. They call this the "rainforest" for a reason. The beaches on the Nicoya and further up in Guanacaste are stunning with ocean sunsets and good surfing, but Santa Teresa takes the cake. Wherever you decide, enjoy...pura vida.

The Pacific coast has plenty of small towns that would answer your need to be "away" but "not too far away". Actually, so does the Caribbean.

Lots of help huh?

Beautiful beaches you will find aplenty on both coasts but the Eastern side would tend to have warmer water. Surfing is better on the Pacific, or so I'm told, I'm not a surfer.

Will you have a car? I would go up somewhere on the mid-Nicoya Penninsula. North or south of Tamarindo.

And as Ruttan suggests Santa Teresa/Mal Pais would be a nice choice as well. Although a few too many gringos for me

Given what you're looking for, I would strongly rcommend Corcovado National Park, on the Osa Peninsula, southern Pacific Coast. It's very mellow, hot, pretty humid. The beaches are beautiful, the water is warm, the wildlife among the best in the country. We spent 4 nights at Corcovado Adventures Tent Camp, and recommend it highly. I don't like hot humid weather, but I would go back there in a heartbeat. You won't be spending any city time, though. there are towns near Corcovado, but only as the bird flies. One you're there, you're THERE.

There are no "amazing" cities in Costa Rica. There are virtually no ruins, and some of the smaller, nicer, more colonial cities have been ravaged by volcanoes or earthquakes. San Jose, the capital, is not nice, and definitely NOT relaxing. Cartago is said to be nice, but I only met one person who went there, and he was not impressed. NATURE is really the buzzword for Costa Rica.

I don't know much about Nicoya. It is mountainous (with the exception of the beaches, obviously), and lots of gringoes go there for the winter. It's also big for surfing. Lots of people like Jaco Beach, on the central Pacific. But I've read it's pretty touristy.

Good luck!
Howie

Only spent 3 weeks in Costa Rica, so i'm sure there is people with more insight than me. But i LOVED Santa Theresa/Mal Pais. I would go back in a hearbeat, really cool vibe, big surfer town, very laid back, just what i was looking for. Also really enjoyed my time in Playa Samara in Guanacaste, another relaxed place, local surfers, really friendly people. Montezuma is also nice, very touristy in town, but we went for a 2 or 3hr walk and found the most beautiful secluded beach i've ever been on, just me, a fresh pineapple and 2 random stray dogs, fond memories, paradise.

safe travels

Corcovado is a little/lot more difficult to get to than all the other places mentioned.
I spent 4 months in Costa Rica from May to September (rainy season) and travelled through most of the country. I loved Puerto Viejo for it's laid back rasta style atmosphere. The beach isn't stunning but there were lots of hammocks and a few horses about. If you want a change of scenery you can rent a bike and head south where there are amazing beaches. From those you can take a kayak and head to really gorgeous and secluded beaches.
The Pacific also has amazing beaches. Manual Antonio is gorgeous but super touristy. One of the most stunning beaches I have ever seen lies in the park. Absolutely breathtaking.
I loved Jaco as well because you can rent a car/bike/taxi and explore some really cool beaches. South of Jaco but before Hermosa we found a beach with a bunch of abandoned structures between a river and the ocean. We spent a lot of time exploring and relaxing in that area. And you can always rely on Jaco for an amazing party.
Nicoya is also great. Tamarindo is amazing but, again, super touristy.
The best surfing is absolutely on the Pacific, but I am a little biased as I am terrified of smashing into the scary Carribean coral.
Hope that helps but I won't be surprised if it doesn't.

I've taken 2 trips to CR, both in May, each focusing on different coasts and resulting in completely different experiences.

The first trip was in 2002 to the NW Pacific Coast with my then-bartender older brother (read: scorched earth policy). We basically made our way to the beach from San Jose through the jungle/mountains/volcano staying at resorts and drinking until we were blind with people we met. I also remember lot's of noise, traffic, pollution, burning trash, scam artists and ugly Americans (like us). From my experience (and you have to remember that i was in college with my booze-bag brother), that side of the country was a lot more like Can Cun than even I remember liking, but maybe that's cause we sought it out.

The second time was also in late May a few years later(i remember this cause both occasions were to celebrate my b-day, the 26th) but had a completely different dynamic. I went with a girlfriend and we planned our trip with the aims of having as laid-back and relaxing a time as possible - we succeeded. We stayed in Manzanillo, just south of Puerto Viejo, and had an amzaing time. There was plenty to do at night, people to meet, scuba, snorkling, beautiful (& secluded if you're looking) beaches and great food. It was quiet, genuine, and completely relaxing.

Our hotel was called The Shawanda Lodge i think, and on top of being great for privacy and accessibility,it was very affordable.

I dont know if you're going alone, with a significant other, looking to be left alone or to meet people, but from the qualifiers you've given, i would HIGHLY recommend following this second path, and suggest you visit the Carribean Coast, going as far SE as time permits.

Let me know if you want more info/details.