Hi,
If you wanted to check out the south part of the Nicoya peninsula, it makes logistic sense to do it after SJO or on your way back to SJO. if you do it after SJO, the ferry would make sense. If you do it after SJO, you may consider just getting transport on the highway and avoided the ferry as that may add time to the trip. The most interesting location in that area would be Montezuma/Sta. Teresa and Mal Pais. Anything outside of that, without a car, will feel a bit remote and you may have to depend on the amenities and services offered by your hotel or Airbnb.
How many days are you staying at your current locations already?
Looking forward to hearing back from you.
Have a great day!