Things to do in Boston/Cambridge

I am going to be in Cambridge for a conference in December (5-11).

What are "Must Do Things/Places/ etc.."

I am on foot this week, so, don't throw sowthing unrealistic. I'll be hostage during the day, but want to see a bit of the town each night.

Todd

That's more New England Xterra Club's territory, not that some MAXC folks might not have some ideas anyway...