Costa Rica is a small country but the roads are twisting and not always in good conditions. Traveling by bus is cheap, but it takes some time. With a rental car you are independent and travel at your own pace and stop whenever you want to. You can look up driving times and distances on this www.yourtravelmap.com
There are some rules that you should follow if you decide to rent a car in Costa Rica. It is dark at 6 PM and you should have reached your destination before that. Some roads are in bad conditions and there are not many signs, driving in the dark makes everything even ore complicated.
Check that your car has a spare tire and all the tools that you need if you have to change it.
Never pay a police officer in cash! They should give you a ticket that you can pay at any bank and are not allowed to accept cash.