My number one suggestion for dinner in Old San Juan would be La Ostra Cosa. It's hidden, so it's not as noisy but the ambience is very nice, the food is good, and every dish has a "passion" rating. There is also a sweet old woman who comes by selling flower garlands. It's at 154 Cristo St, you'll go through an archway and it is at the end of the hall. Lunch can be found on any street corner. There are alot of vendors in front of El Morro. Or if you are there on a Saturday, vendors line La Princessa.
I don't have any reccomendations for Rincon, I never made it past Cabo Rojo.