We've stayed at the InterCity Hotel Frankfurt Airport twice in similar situations and been satisfied. It's cheap, the free shuttle bus runs regularly from FRA, the rooms are quiet and clean, and the beers at the bar are cold.

Friends always suggest we stay in Mainz or Wiesbaden, but we haven't wanted to make it that complicated.

BTW, we're doing the same thing later this year, HKG-FRA-JFK, both flights in F.

David