Use Vamos Car Rental. Comes recommended on TripAdvisor
Get a Kolbi Sim card (best thing ever- use Waze to navigate throughout CR)
Have a GPS, or rent one from Car Rental Company
Waze is highly recommended throughout CR for directions
Get some cash exchanged at airport, there are toll roads right outside the airport, but the toll booths do accept U$D
U$D cash bills should be in very good condition – not torn, faded or folded
Description of roads
San Jose to Arenal: all paved; a dirt road to get to Arenal National Park but nothing terrible. Same story if you want to go to El Castillo (just a bit steep at some points).
Arenal to Monteverde: all paved until Tilaran, then dirt road (they are in the process of paving it, so it is dirt but wide and not too steep). All paved once in Monteverde.
Monteverde to Montezuma: dirt road until the Interamericana junction, but downhill and very wide (super views). Paved from there (both until Puntarenas and from Paquera to Montezuma). Dirt roads from Montezuma to Santa Teresa but nothing unmanageable.