One of the most often asked questions from travelers about Costa Rica besides "How much does it rain in the rainy season?" is: "Do we really need a rental car?". The answer depends very much on you, but by describing a bit more the conditions I can make the decision easier for you!
Driving in Costa Rica is not as difficult as it seems in some frightening reports. Nevertheless it is true that the Costa Ricans that are known for a laid-back and calm attitude seem to get into a hurry as soon as they sit behind the wheel of a vehicle. They are not alone on the planet with this particularity, the only difference is that in many other places the people seem to be always in a hurry.
The roads in Costa Rica are famous for their potholes and some destinations can only be reached on a gravel road. If you plan only to visit San Jose and Jaco you may be fine with a sedan, but it is highly recommendable to rent a 4x4. In the rainy season you will even have to cross some rivers. Even some of the main attractions for tourists in Costa Rica are only accessible with an all-terrain vehicle. The road to Monteverde is not paved and there are no plans for doing so. The local community wants to protect this unique place and does not want that to make their place accessible for huge tourist buses.
The big advantage of renting a car in Costa Rica is, that you are independent and can make a stop whenever you want. Fruits and crafts are sold along the roads, this is always a good opportunity to have some small talk with the locals.
You better get up early in Costa Rica, because the sun goes down around 6 pm. Driving at night in Costa Rica can be dangerous and is not recommendable. Most of the roads don't have lights and road marking mostly exists on the main roads.
The alternative to renting a car is to travel with the extensive bus system. This is a good way to see the country and meet the Ticos. There are buses going almost everywhere in Costa Rica, connections between outlying areas are limited. The capital San Jose is the major transportation hub of Costa Rica, sometimes it is necessary to return and switch the bus here. Before you buy a ticket for the bus ask precisely if this bus really goes to the destination where you plan to go. There are several towns in Costa Rica which share the same name! You can assure going to the right Playa Hermosa by mentioning the Province where you plan to travel. There is a Playa Hermosa in Guanacaste and a Playa Hermosa in Puntarenas. There are several other examples like Naranjo and San Isidro.
Traveling by bus is certainly cheaper than renting a car, but is most often slower. There are several hundred bus companies operating in Costa Rica and unfortunately they don't always work together. This can result in an unplanned overnight stay at a place where you missed your connecting bus.

So the decision is still up to you, but it should be easier for you take it now