Brockenspectre,
I can recommend
www.start.de . I had to get a flight to SYD (a bit far for a weekend, but you get the idea) pronto and they got me out the next day at a fraction of the price others quoted. We've used them to go to other places too, and they never disappoint.
Bizarrely, Iberia's website got us a dogleg to Malaga and back from Casablanca for less than a round trip to CAS!
LH used to auction mystery flights, we took a couple to Italy, to our complete satisfaction.
That said, don't neglect the Vaterland later in the year.
You can make a 'Butterfahrt' sail from Lübeck to Denmark three times in a weekend and splurge on duty free.
The JU52 flight is expensive, but something to boast about now that Concorde is gone.
Good luck, and enjoy your visits to FRA - I was there for five years.
Another tip - get a .de credit card (or access to one) if you can - to make online bookings more straightforward.
BOFH