Steve - Do you happen to have visited LEJ before? I have a vested interest in the area and would quite like to pay a flying visit (excuse the pun) to the airport whilst I'm there. Do you know if they are GA friendly?
Have been to Koblenz and Leipzig, can certainly recommend both as very GA friendly, the rates for Leipzig are reasonable for GA.
Concerning Koblenz, one thing I'd like to point out is that it's final is extremely short (mainly when Runway 24 is in use and you are on the northern right hand circuit - Koblenz allows both Left and Right Hand circuits dependant on where you have arrived from) so be prepared to go around - you need to cut downwind relatively short to ensure you don't overfly noise sensitive areas and lose most of your height on the base leg..... If you're hungry, have a nice greek meal at the airfield, otherwise if you have the time, take a cruise down the Rhine / Mosel - the scenery around that area is pretty stunning. Oh, and sample the local wines ;-)
As a final point, if you are really going to be flying around Germany for a longer period of time, I would recommend you look at getting the "Landegutscheinheft" - these are two books with vouchers (one for the north, one for the south of Germany) which allow you to land at various airports / airfields max 3 times per airfield in 2013 without needing to pay the landing fee - my fellow pilots and I use it to determine where we will fly to at weekends for a jolly and it saves me a packet.....
The list of the airfields which accept it are
here, the costs are €49,90 for each of the books (€80 for both with discounts for AOPA members etc) so if you land at, say, Hannover a few times, the books will pay for themselves.
Unfortunately Koblenz and Leipzig don't participate in this scheme but (eg) Trier and Halle/Oppin do, so they could lend themselves to be alternates as they are close to your chosen destinations......