That is a limited amount of time. I would spend one day driving between certain beaches and stopping at Bar Do Arantes, for example. Mole Beach is one of my favorites due to the waves, but for more serenity, there are other places. Probably a must is a drive to Balneário Camburio. There’s also Centrino da Lagoa (stop at Café Cultura and walk around) and that would make Joaquina beach a must. It's really hard to answer your question without knowing what type of folks you are (e.g., have to see a downtown, fashion, etc.). Also, if you are going now or during the northern hemisphere's summer, you are going to be cold there and I would probably limit beach time. If you have some specific Qs, you can PM me, and I'll try to respond.