Costa Rica: Rental car or private driver?

I went to La Fortuna/Arenal and Puerto Viejo with a rental car and had no issues. I would definitely want a car so you can stop along the way and explore some sites you might not have planned on. Cahuita National Park is an example— it’s near Puerto Viejo and is quite interesting for a couple hours. I would just avoid driving in congested San Jose and Limon which is easy enough.
The usual precaution for car rentals applies- take photos of the car’s exterior both when you pick it up and when you return it.
Have a great trip!

Definitely rent a car and drive. You’ll need a car to get to and around those places and there’s no need to pay for a driver. Driving in that area is no worse than driving in a hilly, rural part of the US or another country. Enjoy the trip!

I found driving in Costa Rica to be quite stressful. Local drivers are fast and impatient. But, that said, we found that having a rental car at times was quite convenient. We've been to the country 3 times and have used tours, taxis, buses and rental cars to get around. The last time we had a rental car, we had a nerve-wracking, stressful drive from near San Jose to Manuel Antonio due to a later than planned departure and unexpected monsoon-type of rain. That visit, the rental company instructed us to remove the radio anytime we were parked overnight (we were showed how to easily remove it). Upon returning the car, we found that the spare tire was flat; thank goodness we did not need it. I've learned to check that now. And definitely take pictures of the car before you drive off from the rental car location.
We really enjoyed the "local experience" using bus travel. It's not the fastest but it's less stressful and we could practice our Spanish with friendly locals. Folks cautioned us to always keep an eye on our luggage, especially when the bus stops for drop offs/pick ups when the compartments under the bus are opened for other travelers. I've heard tales of luggage even been pilfered out of the overhead racks on the bus.
Just my two cents; if you can easily get from your accommodations to tour pick-ups, don't bother with a car.

We picked up a rental in San Jose and drove to all of the places you've mentioned. We had no issues navigating or driving at all. The only stressful part was actually getting out of San Jose proper, but that was short lived. We were advised to use Waze for navigation by the rental company, it never failed us.

We’ve rented cars and driven from both the San Jose and Liberia airports and never had any issues. Some roads may resemble pig tracks, but you just slow down and dodge the worst of the potholes. I recall taking the “back road” from Arenal to Monteverde and being passed by locals riding off-road motorcycles - they were smart enough not to use a car! But without a car you’ll literally be stuck at your hotel needing a taxi or tour to go anywhere (or have to use hotels that are in towns like Santa Elena where you can walk to things). Good luck with the trip!

In addition to all the other comments, you might not want to drive at night since a fair number of the roads are riddled with potholes and even with gps, navigation can have its moments. My family and I (seven of us in two cars) drove to the Monteverde Cloud Forest from the San Jose Airport and had several side trips from our stay there. After that, we drove to Manuel Antonio with more side trips--horseback riding, white water rafting, etc. In short, you need a car. Most of the roads we drove on were two lanes with drop-dead gorgeous views that we just had to stop and get out of the car to take in.
Btw, we rented from Avis at the San Jose Airport which went fine. An "Oh-my-god" experience came with the return of the rental. We were told (and it was in writing) that we were to leave the car in a marked parking spot at the airport, take a photo of the parking spot number, leave the car unlocked with the key under the mat!!! A car thief's dream! It wasn't until over 24 hours later that we received an email from Avis that all was fine.

We loved Costa Rica and still have and wear our pura vida bracelets that I gifted our family with before our flight to CR.

I love Costa Rica and have been three times. I rented a car and had no issues at all. Just know when it’s the rainy season and avoid going then. Rains in the non rainy season can be torrential. Check the weather frequently so you are not out in the boonies in a long downpour.
I’ve never experienced so much sustained heavy rain in my life. It was like a straight down sheet of water and was not during the wet season. My silly son loved taking videos of me soaked to the bone.

I say rent a car. As long as you pay attention, you'll be fine. And don't trust the driving times that Google Maps provides.

My family of 5, plus my mom, went to CR end of November and it was so much more convenient having our own car. As I am sure you know, most of the smaller roads are in terrible condition. They have more potholes than even in PA (IYKYK). And the roads are very dark at night, so try to do majority of the driving before dark. Locals tend to drive pretty fast (or super slloowww) and pass you whenever they want. Find out if you'll need a 4X4. We stayed at a house way up in the mountain that required one. Have a great trip!