You might consider going by bus between cities and using the normal taxi once you get there. You make them put on the meter so you won't end up getting overcharged. You should be able to find a radio dispatched taxi service in Grecia, or at the central park connect with a driver who has a cell phone. That way you could have a regular driver for local trips and wouldn't have to keep giving your address all the time.
Here is a good guide for the public intercity buses:
Costa Rica by BusMost buses charge less than $5 per ticket and you can go anywhere in the country. Most of the time they are comfortable and they are not overcrowded since no standing passengers are allowed.
For tour day trips there are some drivers that advertise in the Tico Times classifieds, you might call them when you get here or check out the online edition ( ticotimes.net )