I would:
-Visit both coasts (try surfing AND charter a fishing trip)
-do national parks (inc Rushmore, Yosemite & Everglades)
-disney world and Cedar Point
-New Orleans (eat at great restaurants)
-los Angeles, San Fran, Chicago, DC and New York (all must sees)
-maybe Boston, Philadelphia, Vegas
-see a nascar race, MLB game and MLS (if summer); consider these options NOT in must see cities.
-museums inc smithsonians
That should fill up four weeks