Frankfurt would not be my choice. Why not research what options are available to you and see what interest you. Be sure to check out if this stop will incur any additional costs (taxes, etc.). It's going to be cheaper to fly into/out of some airports compared to others.
Frankfort wouldn't be my choice either. It's a mostly industrial, not-very-charming option.
I would echo gracie-boo's advice to see what your reasonable options are before soliciting opinions on cities that may not even be an option for you.
I'll third that sentiment by Gracie-boo.
I don't mind Frankfurt as an entry point into Europe (there's enough to see for an overnighter, and the upside is easy access to the city from the airport), but certainly wouldn't go out of my way to spend 24hrs there.Edited: 04 August 2014, 19:13
Amsterdam but I'm kinda bias since I lived there for a while. It's by no means a 24 hour city with everything closing up around 4 in the morning.
Frankfort wouldn't be my choice either
Not sure I'd want to spend my European layover in Kentucky either.
Why not just book an open-jaw flight to city that you want to visit?
Believe that's OP's plan but they don't know what additional city they want to visit.Edited: 04 August 2014, 22:24
I would vote Barcelona, Amsterdam, Paris, Madrid, Prague, London, but much will depend on what sort of routing you have in mind, whether you'd take a train between cities, or whether you see this as a true layover before flying on to Munich. Look for somewhere with a relatively quick and cheap train ride into the city center.
I'll echo the sentiments on not having a layover in Frankfurt. I did that a few years ago, taking the train from Munich to Frankfurt then flying home from Frankfurt, and found Frankfurt to be pretty charmless with the exception of a couple of blocks. Zurich is fine, but doesn't really float my boat. It looks like you would expect it to look, and struck me as a cold city, literally and figuratively.
Have fun in Munich, one of my favorite cities. I'll be there in September as well.
Stopping in Frankfurt allows you to spend the night in Mainz, a short train ride from the airport. You can also see the nearby world heritage fossil site and spend a lovely evening on the riverbank.
if it's with Lufthansa, does he have any other choice but Frankfurt?