jeff, don't have a list of unvisited states, but here are the places they've gone in the US w/seasons in parentheses...
UNITED STATES {Absarokee MT (8 ), Anchorage AK (1, 2), Chicago IL (6), Dallas TX (5), Denver (9), Fort
Lauderdale FL (7), Fort McDowell AZ (8 ), Huntsville AL (8 ), Lake Powell AZ (8 ), Lancaster PA
(8 ), Las Vegas NV (2), Lewiston NY (8 ), Long Beach CA (7), Los Angeles (4), Maui HI (2), Miami FL (3), Middleburg VA (8 ), New Orleans LA (8 ), New York City NY (8 ), Phoenix AZ (4), Salt Lake City UT (8 ), San Francisco CA (2), Santa Monica CA (5), Seattle WA (3), Yellowstone National Park (8 )}