Best way of getting around

Hello

We are stopping here on our way down the Ft lauderdale and will be in port from 7am to 9pm. What is the easiest and cheapest way of getting from the port to downtown around Boston Common or Quincy Market? Any help with private tours or "must see and do" would be appreciated. We have been here for a Bruins game and did the duck tour so we are a little familiar with the area.

Thanks