Our summer trips with the grand-daughters have been in fewer than a dozen states between the south or southwest where SIL has been stationed, and the Great Lakes, where we gather with family. Seven years of doing this, making four week trips, we have always been able to find something new each trip, to fit changing interests as the girls grow.
I think there is a lifetime of things to see and do in just in Texas and across the southwest, but in summer I would be going someplace that offers hope of being cool enough for outdoor activity.
If you've already taken the boy to every state in the lower 48, an he has seen everything there is to see, then I suggest taking him to whatever he liked best. Ask him what he wants to go back to see.
My favorite summer places are northern Michigan, e.g. the northwest quarter of the L.P. with Frankfurt, Interlochen, Torch Lake, Sleeping Bear National Seashore, and Traverse City to Petosky. In the U.P., Mackinac Island, the Soo, and along the Lake Superior shore toward Marquette, and the Keweenau. The girls, however, are happy to just spend a week at the beach at Holland or Grand Haven, though they now live only a few hundred yards from a beach on the Gulf Coast.
I also like the Nature Coast of Florida during the summer, though the grandchildren would just as soon go back to WDW again. At age 11 now.
