I went to Colonia del Sacramento on a day trip from BA. It was historical and worth a visit. One of the few places the colonial architecture is intact. It won't take you longer than 1/2 day to do the walking tour of the historical district/museums. It is a UNESCO World Heritage Site if you are interested in checking those off. Very little museum text/signage is in English, however.

I can't help you out on restaurants. I think I grabbed a sandwich somewhere and ate some ice cream later.

Safe and fun travels!