If it's evening, you can go to the "Top of the Hub" restaurant at the top of the Prudential Center and get a great view of Boston. The restaurant is a little pricey though.
During the day, most people visit Faneuil Hall marketplace aka Quincy market for shopping, dining and some entertainment. There are also street performers in Harvard Square in Cambridge. Easy to get to via the 'T' aka MBTA subway. There's also the "duck boat" tour which you might enjoy riding in an amphibious vehicle going through downtown streets and then cruising through Boston Harbor.
Depending upon what time of day, I may be able to meet up with you for a drink or coffee.