I've been several times and have ridden some incredible trails and have been on a few surfing only trips to CR as well. I took the entire family last trip and used https://www.namutravel.com/destinations/costa-rica to put an itinerary together. Family trip, we started in the Mountains at Arenal near La Fortuna. There are several great resorts.

The Cloud Forest near the gulf of papagayo is also really cool. Further south down the coast, I recommend Manuel Antonio (you have to have passes to get into the park) if you want to see wildlife. The travel agency did a great job pulling resorts in the mountains, where it's more work. I've taken the wife only and it's easy to book a nice place on the coast (four seasons, Andaz, etc.), but it's super helpful to use a company if you want a variety. You can go as high-end or low-end as you want to spend, Bon Appetite,

Bike Arenal. Lookup their FB page. They do stuff all over the country but base out of Arenal. I did the 1968 volcano loop one on one with the Alejandro guy you see on website. Fantastic area and experience. They pickup from the super nice hotels there at the hot springs. He brought me fruit and snacks and a beer for after ride and took lots of photos!

Oh, the volcano one was not for beginners but my cyclocross skills helped me not die.

They also do large groups, long multiday routes, and road rides also.

I would love to go back and do a multi day gravel route.

I go down to CR and bring my mtb every year in February. I like to hit the trails in Hacienda Pinella just outside the big tourist destination of Tamarindo. There's lots of lodging in Tamarindo, and there is a trail that connects the town to these trails. The guy who maintains the trails is a former trail builder from Washington state. He owns a bike shop and rents e-mtb and normal mtb. I have a link to his website here.

This year I'm also going to venture up to Las Catalinas--another popular mtb spot on the west coast of the country. They're actually doing an XTERRA race at this location in 2024.

I spent a day mountain biking in Las Catalinas when I was in Costa Rica. I don't think I'd go there just for that, but it was a quite enjoyable ride with some beach/ocean time later in the day. You'd probably get bored biking in that area after a couple long days.

I like biking, but there is so much amazing outdoor adventure stuff to do in Costa Rica that I probably wouldn't recommend having biking be the main focus every day.

Spent ten days there in March of this year. I saw many people road biking and did see what look at areas that had great forest with trails. My wife and I traveled there with another couple. So for me it was touring the country, eating great food and beverages, I ran only while there.