Meredith, NH and surrounding areas are beautiful in the summer. Take a day or evening cruise on the Mount Washington boat which docks in Meredith.

If you go a little east in CT, there is a great aquarium in Mystic, fun for adults and little kids. There are nice beaches all along the RI coast from Naraganset to Watch Hill.

I have looked into Mystic, but will probably include that in the drive back south. I am hoping for ideas in NE Pennsylvania, and Western Massachusetts and Connnecticut.

For a nice break on your trip, consider picking up route 209 in Strousberg, PA and driving up along the Delaware water gap. It is a nice drive through national park land. There are pathways through the woods that end at beautiful waterfalls. Bushkill falls is commercial and costs money, ( I avoid that place) Dingmans falls and one or two others are park service places and are not commercial. There is a campground along the way. If you get a park service map, you can look for the route along the river that avoids most of the crowded, commercial areas. Try going to www.nps.gov and clicking on PA. At the north end 209 joins route 84.

For NW Mass, there's the MASS MoCA in North Adams, and then the Clark Art Institute in Williamstown.
http://www.massmoca.org/index.php
http://www.clarkart.edu/

Slightly south in Western Mass is the Norman Rockwell Museum
http://www.nrm.org/

In Lenox MA, summer is the annual Tanglewood Festival (Boston Symphony summer home). Lots of fun at the concerts - if you get lawn tickets you can bring your own picnic blanket.
http://www.bso.org/bso/index.jsp?id=bcat5240070

And obviously, the Hudson River Valley in NY state has plenty of see and do.
http://www.hudsonrivervalley.com/

emalloy makes a great suggestion for that time of year. It will be more miles but a very pretty ride. Then aim for Albany and take the route thru VT over Hogback Mountain - you'll be going Bennington VT to Brattleboro. You may want to check some of the historic areas of Bennington. DH likes to stop at the classic car place (sorry name escapes me but I believe they publish a antique car magazine).
This route eliminates going thru CT and MA. You could connect with Rt 91 to go further north in VT, maybe visit Billings Farm Museum, Woodstock and Quechee VT (famous gorge) and then plot a route to Meredith.
Or from Brattleboro, head to Keene NH, take Rt 101 (pretty drive - maybe a short hike in the Mt Monadnock area). I hear a lot of good things about Charmingfare Farm near Manchester NH. It's not too far from Rt 93 then Rt 93 north to Meredith.

Tanglewood is a great idea of you like music. They have a variety of programs, so check their schedule. If that is not to your taste, nearby Balance Rock State Park has a good variety of outdoor activities and trails.

dfrostnh mentioned Bennington, which is an absolutely beautiful town in a mountain setting, lots of walking and exploring there. The drive from Bennington to Brattleboro is pretty spectacular too, and Brattleboro is a funky/hip town with nice shops and restaurants.

From there, dfrostnh has offered some nice options.
The art museum at PSU in State College is terrific, but you probably already know that.
Have fun!

Great ideas! Thanks so much. It is a long drive and having beautiful sights along the way make it much more enjoyable. I just can't wait to get some Penn State Creamery Ice Cream!

If you take the route through the western Berkshires others are suggesting, the Hancock Shaker village, the Mount (Edith Wharton's estate) and Arrowhead (Melville's home) are other good stops.
You could take Rte 2, the Mohawk Trail east in MA, to Rte 91 instead of the Bennington to Brattleboro drive in Vt
http://www.mohawktrail.com/