Acadia is beautiful, but a 7 and 9 year old may run out of interest after a day or two. Every little Maine coastal town, including Bar Harbor has its charm and interesting sights.
For mainland Acadia, parking is a mess and getting there early is very important. I'd recommend getting to the visitor center before 8 and riding the (free) bus for most of the attractions one day, then doing Cadillac another morning. You can only get to the summit of Cadillac by private vehicle (includes private tour buses) or a pretty vigorous hike, and the lot fills quickly. If you like to hike, do it. I still have my sense of superiority from us old farts hiking to the top and watching the tour buses unloading their passengers up there ;-)
The Kancamagus Highway from Conway to Lincoln, NH is worth a drive, even without the incredible fall colors. Several cool waterfalls right on the road or short hikes. Conway and Lincoln are both reasonable drives to the cog railway, which is pretty interesting. Reservations are a good idea for that.
The Lakes Region of New Hampshire is beautiful. Squam Lake is where "On Golden Pond" was filmed. Castle in the Clouds is a bit of an oddball place that is interesting.
Ben&Jerry's is a neat stop.
Vermont Country Stores in Weston and Rockingham, VT might be an interesting half-hour break if you are in the area already. Wouldn't drive far to see them.
IMO, Salem is just a tourist trap. The Boston area has more real history. Lexington and Concord, The USS Constitution, The North Church etc.. Stay well out of Boston and use public transit to get into town. Cape Cod is "nice", but not amazing. Traffic can be bad on Friday and Sunday evening. Fact is I'd plan to not travel near the coast on Friday or Sunday afternoon/evening just because of the mass of people heading out of Boston to get away for the weekend.
Newport, Rhode Island is a very nice day. On an island (with bridges) so it stays pretty cool. Several incredible mansions that will blow you away.
If you want to go further west, upstate New York is a great place. Watkins Glen, Corning and the Finger Lakes region are nice places. Waterfalls, the Corning Glass museum and many other pretty places.
If you want to go even further west, Niagara Falls would be a must-see, but Letchworth state park will be more popular with the kids and is probably prettier but not quite as awe inspiring as Niagara.
You'll run out of time before you run out of things to see.
Plan your route on paper. Your GPS will probably push you very close to New York City, and the tolls and traffic there are tough. Better to drive a little further and stay west of that mess.
Speaking of tolls, an EZPass would be a good investment if you plan to be on any of the toll roads. Mass Turnpike and New York Throughway in particular. It's not just not needing money. In some of those places you'll have to exit to a toll booth to pay, then get back on. EZPass is a big time saver.
