Unless you are used to driving in a major metropolitan city, don't rent. The driving style in CR is what some might call a tad aggressive.

I rent a car about once every 3 trips, usually when we plan on going on a trip for a few days. I hate relying on bus schedules and taxis.
I never rent from the big American chains (Hertz and Budget). I find the prices in the smaller local places a little cheaper. This last time, I rented a standard SUV (Daihatsu Terios) from Toyota Rent-a-car. It was like $40/day + $10 for SLI. They have small cars too. I have also used Advantage (which is Economy Rent-a-car).
They put $1000 hold on your CC. Check with the bank, but your CC should cover CDW (collision damage waiver), but you must pay for SLI (supplemental liability insurance). It runs $8-15/day, depending on car class.
And add a 12% tax if you rent from the airport.
http://www.acerentacar.com/landing/lp_sjoo01.aspx