You should just go for the hell of it......granted, you might not get that much time in FRA (have you checked the bus time/lengths?), but look at it as just something to do to pass time--the best thing is that you get to fly!!
I've just come back from Sweden (see trip report)--and really enjoyed it; I've also flown to Malmo (for Copenhagen), so I guess Torp is next; I'd also like to re-visit Finland--god, I just love their language.....
What I usually do is book at least 3-4 months in advance, making sure I fly Tues-Thurs/Fri; I usually book 2 flights at a time; I've never paid more than 40 quid on any one flight; the guy at a cafe I go to every day just booked his flight to Rome with Ryanair for 80 quid--I thought that was a bit on the pricey side, but he seemed convinced he had got a bargain!
Anyway, a few hours in FRA (or the city) will be great!! Have you visited Germany before? I haven't, I still wanna go to Munich and Berlin.