I recommend doing a triangular drive: from FRA take the A5 northbound and switch onto the A45 towards Dortmund. Switch onto the A4 towards Cologne (Köln) after about 120km, from CGN return towards FRA on the A3.

This tour will be around 450km and will take you through beautiful Hochtaunus, the outskirts of Ebbegebirge in Sauerland and the famous Westerwald. Most of the driving is unrestricted, A45 is 2 laned for most of the drive and quite curvy, A3 and A5 are sometimes restricted, 2 to 3 laned and ensure a great driving experience.

eSixt is renting out a Mercedes CLS for EUR 99/day (don't go for the Audi TT special, the frontdrive TT that Sixt rents out is a crap), Europcar gives you a 5 series BMW for EUR 129 for the full weekend. Both cars should give you enough sensation but you can also rent out a Porsche Boxter from Europcar (great car, even better handling than the "normal" 911 (not comparable to the Carrera series though)).

Enjoy the ride, drive carefully and if you see a 6 series BMW approaching in the rear mirror on the A45 on saturday give way because there is quite a chance that it going to be me...