FF is a fairly big city, not as big as London but though. Hence much to do (in the evenings), but no attractions or something like this. Not very beautiful in my opinion either, although the skyline is pretty amazing at night. Plus, you'll be there in the summer which should be great considering that there'll be one of the biggest Opern Air "WM-Fieber-Parties" in Germany with a "Party-Meile" at the Main. As for security issues FF has one of the highest crime rates (if I remember correctly). However, that shouldn't put you off since it's more about gang rivalries or organised crime rather than mugging or whatever. I've been there only for two hours though and didn't get a real feeling for the place.