You need to define what you mean by "special".

You are right that given how broad your likes are (which isn't a bad thing), and given that it seems you would be happy visiting most anywhere in the Northeast (from Philadelphia to Niagara Falls to New England), you are going to get lots of suggestions. If you can be any more specific now, you will help everyone who tries to advise you.

Explore the history in and around Portsmouth NH on your way to Portland to visit the Portland Head Lighthouse, then go on to explore the mid-coast Maine area. Less crowds,amzing food, quiet beautiful scenery especially around Pemaquid Point and Light house, visit Shaw's in New Harbor for lunch or cocktail at sunset. No crowds. This is the pristine and serene less traveled side of Maine especially if you want to venture past the crowded south coast beaches. Be sure to visit Camden and Rockport towns for more history. Easy 4 hr drive from Boston and 3-4 days is adequate although you will likely hate to leave and want to go back.

Are you flying into Boston? If you didn't want to go to the Maine coast you could visit Cape Cod.
The best way to see Niagara Falls without a car drop off fee would be to take the Lakeshore Limited from BOS to Buffalo and spend the night before getting up early to see the Falls. If you have your passport, go to the Canadian side as well.
You can fly back home from Buffalo.

The Falls are fine but I think other areas are at least as interesting.

However, it does NOT make sense to go to the Falls if you don;t go to the Canadian side - much better views, better hotels, much more pleasant atmosphere than NY.

I had assumed you were driving to Boston, since you mentioned Niagara and Pennsylvania.

Are you driving, or are you flying it?

I like 919CandyW's suggestion a lot.

I might add Cape Ann in Massachusetts -- Marblehead, Salem, Gloucester, Rockport, Essex, Newburyport as Day 1 on your trip.

Day 2 -- Portsmouth, York, Kennebunkport, Portland Head, Portland.

Day 3: Casco Bay mailboat trip, Brunswick, midcoast exploration

Day 4: Freeport (LL Bean) and back to Boston

My first inclination is to send anyone from the Midwest to the ocean - and I think the Maine trip shaping up here is a good idea. I would skip Freeport - it has become a crowded bunch of outlet stores available everywhere and while the original LL Bean is fun, with only 4 days I would skip the shopping and add time to either Portland or Cape Ann. Portland has history, scenery, lighthouses, decent restaurants - all either in the small city or a short drive. The Maine coast is beautiful but unless you are a polar bear, don't expect to be doing any serious swimming.

Gettysburg and Philadelphia is also a reasonable option, but in July might be oppressively hot. If you chose that option, take the train Boston-Philadelphia, rent a car there for Gettysburg and then fly out of PHL.

Boston north on I93 - visit Canterbury Shaker Village just north of Concord NH and a half day kayak trip on the Merrimack River. The rental place will drop you off and pick you up. If you leave from Jamie Welch Memorial Park in Boscawen, it can take a half day (bring a picnic lunch and enjoy a private beach). You won't see any houses even though it's very close to them. A friend took me on this paddle and even though I live in the area, it's a completely different view, different birds, people enjoying one of the many private beaches.
You can head to Portsmouth via I393 to Rt 4. I have had one complaint of too much traffic in the area of all the antique shops (watch out for sudden stops) but we've never had a problem even when pulling a camping trailer. This route will take you over the bridges that go over Great Bay. Just beautiful. From Portsmouth you can take an inland rivers cruise/narration that goes up thru this area. Get out on the water as much as you can!
To avoid so many hotel changes, you could stay in Portsmouth and make a vist to Portland ME a day trip. Do a foodie tour of the Old Port Area. Take a mailboat cruise. You can visit some beaches the followin day - Kennbunks, Wells, Oguiquit, York.

Normally I dont recommend southern Maine even though it's very popular. Rt 1 traffic will be extremely heavy so don't visit this part of the state on a weekend. From Portsmouth it's only about an hour back to Boston on I95.

Dfrost:
I LIVE in New Hampshire and I've never done most of your suggestions. You've given me some great ideas for field trips this summer. Thanks!
Gail

Gail, if you're old enough, there's an adult ed program called OLLI or something like that. The seacoast chapter offers some interesting boat trips - one on a gundalow. We haven't been on any of these trips but I have my eyes open for something like that. The Hippo Press has a pretty good listing of events. Sometimes a really great local event is not well publicized so there's no crowds. DH doesn't do crowds unless nagged and negotiated.
We just need summer to get here!

dfrost:
I don't want to hijack the thread, but just to reply, yes, I'm WAY old enough for adult ed, will check out OLLI and also Hippo Press. We love field trips. muchas gracias.

If it's gonna be Maine or Cape Cod in July you'll need to book something in the next few weeks. Since you enjoy history, I'd choose Gettysburg area and Eisenhower Park.

http://www.nps.gov/eise/index.htm

Fabulous suggestions! We could either drive or fly...depended on what we decided to do. Thank you. Now we have a starting point to plan!