Best ways for getting around the country?

I’m thinking about going to Costa Rica at some point over the next year. While I know where I would like to go when there, I’m unsure about the transportation logistics between destinations. I’m somewhat leery of “small planes” (propeller style planes and assume there would be quite a few of these in Costa Rica) but am not sure if it would be better to rent a vehicle myself or hire a driver. I’m hoping to visit Guanacaste, Arenal, and Puerto Viejo de Limon. I read somewhere about the company Interbus that offers small group transfers and seems decently priced. How would you recommend getting between places? Thanks in advance for your response!