Harvard Square is great.
Check out the Brattle cinema for cutting edge movies, not your usual run of the mill movies, and get a bite to eat @ Casablanca's.
Great for people watching. The COOP bookstore has a great selection of books, new and used, plus a bunch of ""cheap discounted"" books!
I have brought my kids here, but not at night!
Pros: Brattle theater, Coop bookstore, people watching
Cons: snobby people, no parking, can get crowded