I would check out Guanacaste area. I found it to be beautiful there. It’s near the city of Liberia which has a population of 40000. It’s also the most touristy area so if you like beaches, mountains, and nature stuff that is where to go.
I am not sure how good the infrastructure is there unfortunately.
I once stayed at a resort in Nicoya and there was black sand beaches there. There’s also traditional white sand beaches all around that area.
If you want to be close to San Jose I think there’s decent areas that are about an hour away. I’m not sure if that is too far for you or not.