There is not a lot in FRA re: sightseeing. It is the financial centre for Germany and specialises in exhibitions and expositions so there are loads of hotels.

I know many of my Germany colleagues commute to FRA during the week. It may be worthwhile taking a day trip to somewhere like Hiedelberg which will have more traditional character.

A taxi into town is about 20 euros and relatively hassle free. The airport is not too far from town. Road infrastructure is good.

No immigration fast track in FRA T2. Not sure about T1.