We just connected through SJU last week on United. You can access D concourse inside security from A, B and C if you chose to pay for Admirals club. I recall there's only one sit down restaurant in C, but they will give you wifi code while you eat/drink. Also free wifi at the gate opposite Starbucks in C (Network name has the word "blue" in it, but can't recall the entire name).

Not much to do at the airport, would agree with everyone else that you spend time elsewhere and minimize time at the airport.