What's the easiest way to travel around Costa Rica for a non-Spanish speaking couple?

I don’t speak Spanish and had no problem driving all over the country. I didn’t drive at night except to go out to dinner. I also didn’t go during the rainy season.
Most hotels can hook you up with day tours.
Many years ago there was a bus system that made scheduled runs between hotels/resorts. I’m sorry I don’t know the company name. We stayed at several hotels and often ran into the same people. They used that bus system to get around the country.

We were there in February for the first time. We rented a car and had no problems driving. We did take all the additional insurance as we didn’t know what to expect and we were concerned about collisions and theft. We had no problems. Kept doors locked and nothing inside. We drove from San Jose to Manuel Antonio/Quepos 3-4 hour drive and did trips from there. We did not drive all over the country. The car had gps and we did add WiFi to the vehicle, so we could use our phones also for Google or gps. I had not purchased or planned for international roaming on my cell phone, so I didn’t want to use cell data too much. The WiFi device was portable, so we could take it with us and use WiFi anywhere. Really didn’t need both the gps and WiFi, but we wanted a backup plan while driving, as we didn’t know what to expect. Where we stayed had free WiFi also.

Many of the day trip tour operators/excursions offer free pickup at your hotel or wherever you’re staying. We took advantage of that a few times so we saved gas and didn’t have to hunt for the tour starting location.

English is widely spoken. We don’t speak Spanish and had no issues. If you’re staying in the tourist areas, English is spoken all over. I don’t think you’d have any problems with taxis or buses due to language. They are used to tourists.

It’s a beautiful country with lots of wildlife. We had a great time. Enjoy!

You don't say when you're going, how long, or what areas you hope to see. But if you're just talking about a 7 to 10 day trip, staying in 2 or 3 places max, you don't need a car. As alluded to above, there's tons of shuttle vans - one is Grayline of Costa Rica - that have scheduled daily group departures between the main destinations. If you're a party of two or more, it's not much more to take private shuttles and leave the driving to someone else. Distances don't seem that long but the roads are not that great and the driving takes way longer than you think it will.

We were there in January and used Morpho Vans for private transfers from San Jose to Arenal, Arenal to Manuel Antonio and then MA back to the airport. They have great reviews on TripAdvisor but I only found them to be aqequate.

So least stressful? Private vans, I'd say, but we could use more details about your trip.

@ Valerie Yes, mostly likely a week. We're most interested in seeing the national parks and the cloud forests.

Another vote for private vans/private transfers. Easily arranged - though we had a travel agent do it for us - and inexpensive.

@zidek So all of your transportation was arranged prior to arrival?

For a week, just choose two places and do day tours from there. All day tour operators I have seen come and pick you up at your hotel. It's typically a bus that seats about 14 people. They are a little tight, in my opinion but it works. I booked our transfers before we left. Morpho Vans was fine but they had one driver that texted while he drove and I asked him not to and he stopped. All other drivers were fine but two of them brought along a passenger on our private ride - one brought his wife, which was fine. The other brought an unknown woman, who rode with us for 5 hours. She was friendly, but she tried to sell us jewelry at some point;) Private transfers gave us way more room to stretch out and relax.
If you haven't picked your two locales, that should really be your next step.

@tph2010 yes all transfers were arranged in advance. Didn’t leave much room for itinerary improvisation, but a fantastic trip nonetheless.

@Valerie How far were your hotels from the airport? How did you get to them from the airport, and how did you get between hotels (if you stayed at more than one)?

This was my second trip to Costa Rica and I knew I didn't want to stay in San Jose at all. So when we arrived from Seattle at about 4pm, I had Morpho Vans waiting for us at the airport. They drove us right out to Arenal/La Fortuna. That ride took about 4 hours. We then spent 4 nights in La Fortuna. While there, we did day trips - one river ride that took all day, one Don Olivo Chocolate Factory tour that took 4 hours, one trip to Proyecto Asis wildlife refuge that took 5 hours. We then had Morpho Vans drive us to Manuel Antonio. That ride took about 4 hours. We spent 3 nights there. From there we did a half day hike of the national park and spent the rest of the time there relaxing. We had an afternoon flight home on our last day and so we had Morpho Vans pick us up early and drive us right back to the San Jose airport.

I have a blog that includes details and pics from this trip. PM me if you want a link.

@Valerie Thank you for all of the information and advice, I will certainly look into Morpho Vans.