Have been to and recommend: San Francisco, Seattle WA, Boston, Cape Cod MA, Bar Harbor and Portland (Maine), Newport and Providene RI, Atlanta GA, Savannah GA, Charleston SC, Myrtle Beach SC, Baltimore MD, Annapolis MD, Philadelphia and Lancaster PA, and the Salt Lake City Utah area. Vancouver has a lot to see and do too.
Places that are on my list to see: New Orleans, San Antonio, Rapid City SD for Mount Rushmore, Chicago, New York, Memphis for Graceland and more, Lexington KY, St.Louis, Oklahoma City, Denver, Portland Oregon, Santa Fe NM, anywhere in Wyoming and Idaho. So many more cities!
How great to take 5-6 months and just see as much as you can! Good luck planning!