You can spend an entire day exploring the thousands of works of art at the MFA and still not see everything. This is an excellent way to spend some free time regardless of weather.
If you're in town for more than the wedding, make some time to see the best baseball team in the country play. Red Sox will be in town against the Rays June 9, 10, and 11.
Check out the Freedom Trail, a 2.5 mile route through Boston that highlights important national historic sites and museums.
Check out all the shops and restaurants near Quincy Market.