If you usually stay at a Marriott, why would you stay at the airport Sheraton when there is a great Marriott in the city? Ask for a room with a view.
The city has so much to offer as far as sightseeing, food, markets, and museums. The trains to the airport run every 15 min. and only take 11 min. to get to there.
Visit the Farmers Market on the Konstablerwache (9-1700), in Bornheim on the Berger Strasse (8-16:00), or in Höchst (7-13:00) which is part of the German Half-Timbered Route. Have lunch sitting up on the city walls, admiring the palace and then visit one of the oldest churches in Germany, the Justinus church built in 850.
The flowers might be in bloom at the Palmengarten, there are some great exhibits at the museums especially the Schirn (Magrite) and the Städel (Battle of the Sexes), but you might want to take time to visit the Judengasse Museum with the Jewish Holocaust Remembrance Wall next to it. Stop into the Klein Markt Halle for pastries or sausage, go admire the wall paintings in the Karmeliter Cloister and have dinner in an applewine pub in old Sachsenhausen. (Dauth-Schneider or Atschel)