as long as you have an American passport go through immigration and take the metro in Terminal 1 to visit downtown Frankfurt. It's a drive of only 20min.
Frankfurt isn't that big so you are through within 1-2 hours. Just take S8
direction "Hanau" or "Offenbach" and get out at station "Hauptwache". One level upstairs and you are in the middle of Frankfurt.
the shop you are referring to is in arrivals hall B right accross the exit B2
named "Relay". But they don't have so much variety anymore - only books (mostly in German), postcards with airplanes and some Herpa mockups.
if you want to stay at the airport at Terminal 2 where the McD. located is there you'll find the only visitor terrace of the airport. during 25R/L operations it is very recommandable as you will see all arrivals and departures except the ones from runway 18. also you will have an overview of Terminal 2 and the apron infront of the terminal.
Terminal 1 in terms of aircraft spotting is very bad. only after security there are a few spots with good views e.g. gate A26/A28, gate A1 (a little bit). but there are better restaurants than in terminal 2.