Comm Ave does not run in Cambridge. The name you're looking for would be Massachusetts Ave.
jn: you definitely don't want to be driving around here. The streets could be hard to navigate, with weird alignments, lights, one-ways and no-turns, and the biggest problem is that parking is expensive and often restricted to residents in most neighbourhoods. Transit is convenient enough for most purposes (except for the occasional unexpected breakdowns, and during Sox games), and if you're a transit geek you'd like to ride on the first subway in North America, and get a feeling of what Transit City is going to be like.
As for things to see, it really depends on what you're into. Most of what HD suggested are good. If you're into architecture, Beacon Hill is beautiful; Back Bay and esp. South End are excellent for checking out the rowhouses; Southie and Mission Hill have many beautiful wooden triple-deckers; and Charlestown have a bit of both. Bulfinch Triangle and Leather District, for 19th century office/light industry buildings. North End is Boston's little Italy, and much better than Toronto's IMO. Definitely check out the Italian restaurants and desserts. I am a bigger fan of Mike's Pastry than Modern.
If you are into institutions, the Harvard and MIT campuses across the river in Cambridge are worth a visit (MIT also has some interesting architecture). Museum of Fine Arts has a pretty good collection; Institute of Contemporary Art for, well, comtemporary art (or just to look at the architecture of this building); and the Museum of Natural History at Harvard is excellent if you're into fossils and animal specimens. If you're into history, follow the Freedom Trail through downtown to visit all the colonial and revolutionary buildings and sites. The Charles River Esplanade is good for a jog, biking, sailing, etc, and shows you how a city can utilize the waterfront correctly (despite of the highway that cuts off the Esplanade from the city). If you don't get seasick, take a boat ride to the Islands, or out to the Massachusetts Bay for whalewatching.
Since you have a car, drive around to the many coastal towns in MA, NH and ME (Gloucester, Rockport, Portland, etc) for the chowder, lobsters, seafood and clam shacks. You can get pretty good ones in Boston too, though it could get expensive (eg the local chain Legal Seafoods; Union Oyster House, the oldest operating restaurant in the US, is worth it though). Cape Cod and Provincetown (with a pretty vibrant LGBTQ scene) would be beautiful if you have at least a day to spare.
Avoid doing the Duck Tour; it could be cool to ride the amphibious trucks in the river, but at $30 it's pretty much a rip-off.
