Cambridge, MA has one of the best concentrations of great eateries of any place I've ever lived; here's the list of my favorites, dated from the last time I sent it along to someone. (I know at least Herrell's is gone, which is a crime against humanity.)
Darwin's: my favorite place in Cambridge. There are two of them in Cambridge, one on Mt. Auburn, one on Cambridge St. The one on Cambridge St. is a bit bigger, but generally a bit busier. Get a sandwich, get coffee, and get a coconut macaroon. Eat the macaroon while it's still cold. It's unbelievable. (I recommend ordering at least two macaroons per person, based on personal experience. Last time I was in Boston, I brought home 12 of them … and we'd eaten them all in less than two days.) They also have amazing breakfast food, but only if you get there early enough. If you only go to Harvard Square for one meal, this is it. The last time I went to Boston, I had the cab drop me off at Darwin's. (If you do only have one meal in Harvard Square, don't miss Herrell's on the same trip.) I have only had one bad experience – Asia's first sandwich there was on really bad bread (I think it was too dry, but I don't remember what kind it was). We went up and got her a different kind of bread, moved the sandwich over, and it moved up to one of the five best sandwiches she'd ever had.
The Helmand: This is also in Cambridge, and easiest to get to by driving. Make a reservation, but it's not expensive. In fact, given how amazing the food is, it's probably the best deal in Cambridge. Everything on the menu is amazing. Not in the best part of town.
Herrell's Ice Cream: This is some of the best ice cream anywhere, ever. They tend to make weird flavors (like “earl grey”), which you should taste if they sound at all good. People in Boston will try to tell you that JP Licks is better, but they're just plain wrong. The hot fudge is not to be missed, if you like chocolate at all. If you don't like chocolate, consider upgrading your taste buds! :)
Veggie Planet: vegetarian pizza that's fantastic. If you like spicy food, I highly recommend the Unsafe ‘n’ Sound, or the Safe ‘n’ Sound if you don't. There's a Mexican bean pizza that's also pretty darn good, I think.
b. good: A “healthy” burger joint, which is pretty damn tasty. A much lighter meal than Mr. Bartley's, and better in several ways – much more unique, to say the least. Look at the left wall when you're standing by the counter to order – there's a “monthly special” which is usually freaking fantastic. This is (to date) still the best white bean burger I've had anywhere.
Mr. Bartley's Burgers: Huge, tasty burgers. Go at an off hour if you can, because it gets really busy and noisy. Don't miss the sweet potato fries.