As you don't have much time you could fly to Drake Bay or Puerto Jimenez (at least one way). By bus it is 8 hours or so from San Jose to Puerto Jimenez. Drake Bay would be 6 hours or so by bus (bus to Palmar Norte, bus or taxi to Sierpe) and then a boat to Drake Bay. Do you have 7 full days or are you including your day of arrival and departure?
If you could spend 5 full days in the Osa you would have to choose: Puerto Jimenez, Cabo Matapalo, Carate and maybe the three day trek to the Sirena ranger station in Corcovado Park.
Or Drake Bay with a boat tour to Sirena and the snorkeling tour to Cano Island.
As for Cahuita: I would actually choose a beach south of Puerto Viejo to stay at if you are going to stay in one spot. That is also where you would rent the bike. And do a day trip to the Cahuita National Park by bus. Again, it depends on how many full days you have. If you do stay in Cahuita you can choose to be near Playa Negra (a 15 minute walk from downtown and the National Park) or downtown.
