I did 8 days hiking/ecotour through Costa Rica in January. I have to say it was the best time I've ever had traveling, and I did it solo. Along the way I met plenty of folks and had a blast.
For safety's purposes, a few pointers:
San Jose can be very dangerous for tourists and locals alike after dark. As MG said, spend as little time there as possible. I'm sure Omid could, and I will definitely suggest that when there, stay at Hostel Pangea (www.hostelpangea.com
). They offer a shuttle from the Juan Santamaria Airport (if you fly to San Jose, its where you'll be) for at most $14 pp. If you get lucky, you'll be riding in with several people, and the cost drops to $6 or so pp. (email them ahead of time and reserve a spot, its very popular and that way you make sure the shuttle is there) Cabs are around $20, and you do not want to take a bus in. The bus stop is in a BAD part of town.
Spend as little time as possible there, and if you do go out at night, taxi everywhere directly- do not walk around by yourself at night- it plain isn't safe.
Ask the hostel desk folks about what to do/where to go/where not to go and pay close attention to that. Once it got dark out, I didn't venture outside Pangea (there wasn't really a reason- they had a bar and little soda on the roof.)
Beaches- I cant really specifically say any over another but I do know that a lot of friends I met had a blast in Tamarindo and Manuel Antonio on the Pacific Coast, but they said they were also VERY touristy. I did meet someone that was mugged at gunpoint in Tamarindo, however, so use caution.
FWIW, the Ticos I talked to told me that the majority of the crime comes from poor Nicaraguans and Panamanians coming into CR because the economy and standards of living are so much higher there than at home. They also said parts of the Limon province are pretty rough, too (Carib. side), specifically Puerto Limon.
Exercise caution- don't leave your bags unattended, don't wear flashy jewelry, advertise your iPod, camera, etc etc and you'll be okay. Use common sense.
If you are interested, I doc'ed my trip on my blog... Misadventures of a Crazed Kitchen Pirate
Any other questions and we'll be more than happy to answer them!