When will you arrive datewise and on which airline. The advantage of Sheraton, Marriott, Hilton and Hilton Garden Inn are not so much of an advantage if you do not arrive with Star Alliance in Terminal 1. The Holiday Inn is closer to Terminal 2 and next to the S-Bahn Station Gateway Garden, which offers a direct connection to both Mainz and Frankfurt Downtown. None of them offer view towards tarmac, but you might ge views of planes approaching Frankfurt. If you are into planespotting, I suggest to have at least a drink at the rooftop of the B'Mine Hotel (belongs to Worldhotels).

Downtown Frankfurt has a lot of different Museum (Schirn Kunsthalle , Städel Museum, Jewish Museum). Depending on food, there are plenty of options in Sachsenhausen