If you're spending some time around Harvard Square, don't miss Grafton Street. It is a nice restaurant with a good atmosphere, nice terrace, very friendly and quick staff, really good honest food and all at a very reasonable price. We went there for a late breakfast and were not at all disappointed. The coffee also had a decent strength, something that you don't always find in Boston.
On a sunny day the terrace will be packed, but it has a good number of tables and there is a good flow of people coming and leaving, so you don't have to wait that long if it's not 'prime time'. And inside is pleasant as well, both on a sunny day with the big windows open, or on a rainy day, gazing at the people passing by.
We were there for a late breakfast, but I could also see us hanging out at the bar with a red wine and a good local beer. It's a place that transforms itself throughout the day.
Very convenient location, close to Harvard bookstore, Harvard campus and red line stop. And nice staff. Did I mention the nice staff?! Worth a visit. Don't miss it!