As far as I know, Thrifty generally has very reasonable rates (well, in comparison to the others at least) but Sixt and Europcar also have some valuable deals from time to time. National, Hertz and Alamo offer rental cars at FRA
as well but I don´t know anything about their rates.
As other people have already pointed out, if you and your dad will stay at Frankfurt, you might save time, money and nerves by choosing public transportation. However, if you want to get to the famous spots at the A5 and runway 18 or to Speyer/Sinsheim, a car would be helpful.
Btw, cities like Cologne or Stuttgart can be conveniently reached in almost not time by taking the ICE train (booking in advance can save a lot of money).
I hope that I´ll be able to participate in the FRA
meet as well, things are looking quite good at the moment.
And to some certain members: Please feel free to bash my spelling as much as you want but IF
you do, check your own "abilities" first