Our life goal (within five years) is a house in Nicaragua. I want a courtyard and warmth. I've never had a problem driving north or south in Costa Rica, or driving down to Nicaragua, but again, you have to pick a good car because most of the roads suck. Don't be afraid to drive. It makes the trip a joy.
THIS. I have driven through every Central American country except Salvador, and only skipped that as they were busy shooting each other. Costa Rica is EASY with a 4 wheel drive and doable if you are a decent driver with a beater and have some common sense... having said that, I have been to CR a few times and didnt feel the need to have a car, as buses/short flights to fun places and cabs are available... negotiate your price up front and if you need to, have them write the price down BEFORE you GET IN if you are not sure of your numbers... trust me, this will save you a shit ton of annoyance/$$$.
Corcovado National Park was amazing if tough to get to... tons of animals however, but primitive digs to stay in, plenty of small places right on the beach to stay in at decent prices... you will have a blast.
