Itinerary Help-must dos and beaches?

Hi folks!

My husband and I plan to visit in January for 5-7 days. We are going to rent a car (have seen enough threads on this to know to be careful with insurance and whatnot) but mainly I want help on which beaches to visit. We are in our mid-20s and more the adventuring type. Neither are big drinkers so I have no interest in the party spots. We plan on spending the first half of our stay in La Fortuna and open to ideas for the second half. TIA!