IMO, the best way to rent a car in Costa Rica is to NOT. They have the highest highway fatality rate in the world, although I don't know if that is per passenger mile or per capita. The roads are in terrible condition and the signage is poor to non-existent. Get to Rincón by flying to Liberia. I do not think there are direct flights from the US to Liberia without going through San Jose but there are charter flights direct to Liberia from many cities. It's relatively cheap to fly within Costa Rica and rental cars are hideously expensive. You really should have 4-wheel drive which will drive the cost up further. Insurance is very high as well. We are talking well over $100 per day before you consider gasoline. Gasoline prices are set by the government and worked out to about $2.60 a gallon when I was there in Feb. A lot of people have diesel cars as diesel fuel is somewhat less expensive.
Doakes