I've been to Costa Rica twice and absolutely love it!
In terms of rental cars - it's true that if you look at typical rental companies, the quoted price ISN'T what you'll end up paying (due to government mandated insurance). However, check out Wild Rider rental (
http://wild-rider.com/) - absolutely impeccable service, perfect little 4x4 two-door, honest price for the area (600USD for 11 nights, tax, insurance included). Since I wasn't there for a long time, I didn't want to waste entire days on public transport (buses are efficient and cheap, but not as quick to get from one place to another).
If you go to Arenal (which you should!) take a hike up Cerro Chato. You can get a view of Arenal and the interior crater of Cerro Chato from there. We stayed in a little studio in Nuevo Arenal (19$ USD a night), which is a little village with very few tourists (we didn't see any).
In that area, go to the free hot springs, just across the street from Tabacon Resort. Walk under the yellow gate across the street from the resort, down hill a few yards, and you'll land in the free hot springs where the locals chill.
Viento Fresco is a great little day-hike as well, where you cross paths with half a dozen little waterfalls. It's just a little south from Tilaran.
La Catarata Llanos de Cortés is a lovely little secret, just a little past Bagaçes. You'll go a few km past the town, turn into a dirt road, pay a "contribution" to the local church (on whose land you are - I think we gave 2 USD and the women gave us some fruit in return), and you'll end up in an amazing little oasis with a huge waterfall. You can even camp on the beach there.
Then, we ended up in Nosara on the Pacific coast for a week of surfing. Nosara, because of the surf/yoga tourism, is a lot more exensive then the other areas we visited, but Playa Guiones was awesome and some great waves.
Just an FYI, most of our in-between activities (those that I mentioned plus way more), we're suggested during the 2 hours that the guy from Wild Rider spent with us, map on the hood of the car, asking us in what areas we were staying, what the best route would be, where to stop, how to find less touristy spots, etc. Half of what we did we wouldn't have known about, which is another reason why I encourage and share the name of this local car-rental company (owner is a European expat - Danish I think, all employees are locals and very helpful, before, and after the rental).
Feel free to PM me for more details if you want!
Have fun!