Arenal and Manuel Antonio are must sees. The hot springs around Arenal are incredible - I went to Tabacon hot springs and was in heaven. Also zip-lined/repelled in this area, great times.

The beaches in M.A. are great (national park where you can see spider monkeys running around on the beach), and you can get some cool souvenirs on the main road there. Nice place to kick back. On a side note, tons of stray dogs running amuck there. Dont feed them (I did and almost lost a finger).

I also went to Monteverde Cloud Forest. Unless you are a serious birder, I'd skip that area though.

As for the driving - just take the point to point shuttles. Roads in CR are terrible (they call it the costa rican massage) and the drivers are nuts. Plus CR is a beautiful country, you definitely want to soak in the views on the road.