DeCordova Museum is a little outside of Boston.
Part of it is an outdoor sculpture park, lovely if the weather is cooperative. Nice place for a picnic -- grab some sandwiches from some sub shop ahead of time, if that works for you.
There's Fireworks & Boston Pops & howitzer/cannon/1812 Overture at the Esplanade if you are actually here on 7/4. Enjoy crowds?
Highly recommended if you enjoy fireworks and the weather is reasonable. It's possible to spend all day there. If you want seating up close, you have to go quite early. The Esplanade is on the Boston side of the Charles river. The fireworks themselves are shot from barges on the river, and so are visible from the Cambridge side as well. On the Cambridge side, Memorial Drive follows the river, and the two directions are separated by a wide grassy island, which also makes for great viewing of the fireworks. I don't know if they do it every year, but I've seen in the past that they have at least on occasion set up viewing screens and speakers on the Cambridge side so you can see and hear the concert. Note: if you're right by the Hatch Shell for the Pops, it's not necessarily the best viewing spot for the fireworks. Anyway, weather permitting, it's possible to spend the entire day there, and have a nice relaxed day. If, as CrossRoads said, you don't mind crowds, because it does get very crowded.
You-do-it Electronics in Needham - will seem overpriced compared to dipmicro tho.
If we have any surplus electronics supply outside of computer places, I don't know about them.
I certainly enjoy You-Do-It, but if you're used to ordering over the web, it won't seem anything special. There are the occasional deals. It's just that there's not much else around, retail-wise.
Do you remember Eli Heffron in Cambridge?? I don't understand why they went out of business. Probably just high real estate costs there. But I miss it, even though I only visited a handful of times.
I recommend using the subway if you're moving around in the city, as it's pretty convenient. Parking can be a hassle, and quite expensive, unless you know some good secret spots. I don't, or I'd share. I live about 2 miles away from the end of one of the subway lines, and I would never drive into the city. Buses aren't bad either, for getting out of town, the system is pretty extensive, but parking isn't so tough to find or expensive out in the burbs. So it's possible to fly in and use public transportation the whole time, if you want to keep it inexpensive. There are visitor's passes for public transit, too. There is a subway station at the airport (accessible via a shuttle bus), which isn't too hard to deal with depending on how lightly you pack. Or course, with public transit, you are at the mercy of someone else's schedule, and you will have to walk more than you otherwise would. But if those constraints don't bother you too much, it's worth considering.
Hope you enjoy Boston!