Your US license is good down there. They do NOT care if you have insurance from home that covers you down there. The roads and drivers are AWFUL so buy the extra insurance. There are no road signs off the main roads, rent the GPS unless you have an app for your phone (AT&T roaming data rates got a lot cheaper last year).
People in Costa Rica do NOT like to say "I don't know" so if it seems like their directions are iffy, ask someone else too :)
Don't leave anything in your car, every one says that, rental places and residents. Don't drive at night.
Car rental is VERY expensive down there. Buy a copy of the TicoTimes.NET before you go (get the latest PDF) and the classifieds in the back lists some car rentals from local places.
FYI, Interbus and Greyline does shuttle busses around (8-14 person vans) and so do local places like montezumaexpeditions.com (I love these guys) for getting around (usually they are door-to-door). I use them for getting around most of the time. almost 8 years ago