I'm not doing any traveling in Costa Rica this time around but but thought I'd chime in just to say that on a previous year I did rent a car and tour on my own after a conference and it was great!
I felt perfectly safe (even when I got a flat tire on the highway at dusk) and had a lovely time. I went to Grecia, Atenas, and San Ramon (stopped in Sarchí), there was a ton of info on Facebook expat groups, makes it easy to ask/find out about expat gatherings/hangouts. Prior to that I'd been to and enjoyed beaches around Manuel Antonio and Ojochal. Went to Limon on the Caribbean side many, many years ago as well (there's a nice little expat community there now).
No matter what part of the country you choose, it's hard to go wrong. The number one thing I personally do to ensure a good trip is to make some connections via facebook ahead of time and number two book stays at places with great recent reviews.
Best,
IL Overseas Editor Jess Ramesch