We'll be getting into Frankfurt airport at 6:00 AM and leaving about 11:00 to return to the States. Is this enough time to see anything around Frankfurt, and what would that be?
Not a lot of time (assuming that your flight departs at 11am) but you could take S8 or S9 train to Hauptwache and have an extensive German breakfast at Cafe Karin (www.cafekarin.de) right across from Goethe's house.