Frankfurt

Will you be flying out of FRA after your 1.5 days, or taking the train somewhere? There are a few satellite neighborhoods/settlements that might qualify as "old-town" zones, places where you might want to stay, but you'd want to be situated so that your departure isn't difficult.

IMHO, there is not a whole lot to see in Frankfurt. Depending on your itinerary,
maybe head to a place like Mainz, Heidelberg, or Wiesbaden, which are smaller,
more charming, and still easy to get to Frankfurt Airport from.

I haven't done it, but the Frankfurt-on-Foot walking tour is highly recommended by many.

I am also going to recommend the Frankfurt on Foot walking tour.

If you like historical sites and old town, this is the tour for you. I had the pleasure of taking it in November and loved it.

I thought Frankfurt was a modern, banking city. The tour showed me otherwise. Lots of history.

There are a few significant historical sites.

• Goethe home -https://maps.app.goo.gl/jwMpJupQTfGw8UX18
• Frankfurt Parliament site

There are also some Roman ruins.

Historical sites in Frankfurt:
* Kaiserdom, where the Holy Roman Emperors were elected starting in 1356, and crowned starting in 1596
* Judengasse Museum built on top of the former Jewish Ghetto - 1462-1800
* Jewish Holocaust Remembrance Wall (you can go find Anne Franks name as well as her sister and mother) the medieval Jewish cemetery it encloses, used from 1272-1828
* Deutsche Orden Church - 1309
* Kleinmarkthalle - great market hall for produce, pastry, cheeses, meats
* Alte Nikolai Church - 1290
* St. Leonhards Church - 1219
* Frankfurt Höchst - part of the Half-timbered Route, small schloss surrounded by a dry moat, St. Justinus church, one of the oldest churches in Germany as it was built between 830-850, great farmers markets on Tues, Fri. and Sat. from 07:00-13:00 plus a market hall
* Frankfurt City Forest - 11,000 acres of woods. Go climb the Goethe Turm. Lots of water fowl living in the lakes here
* Frankfurts new Alt Stadt - finished in 2018, located where the former old town used to be
* Alte Oper - built in 1880 and one of the most beautiful opera houses in Germany
* Palmengarten - Botanical Garden
* Franconoford - archeology right in the middle of the old town, with Roman bath ruins, palace walls from the 800s, and medieval cellar walls.
* Karmeliter Cloister - largest religious wall paintings N. of the Alps, painted between 1513-1521

I don't have a family run hotel recommendation as a I tend to use Marriott and ibis chains due to their consistency from city-to-city. The Pure is a Marriott property near the north entrance to the train station that is OK. However, I suggest your hotel be close to or in the old town area, rather than near the train station front/main entrance. The RLD is right around the corner and the main streets coming out of the station are filled with the homeless and drug dealers. Even though there are very few crimes against tourists, it isn't the most comfortable area to walk through at night.

In addition to other recommendations provided in other posts, I will add the Stadel Museum as a place to see.

For hotels, try the Ruby, Motel One on the Römer, Moxy, Flemings City Center. There are some smaller hotels in the Westend that might be good for a family, Hotel Beethoven for example.

I would not choose the Pure, simply because of the location. Coming in from the airport, you can stay on the S-8 or S-9 and get off 2 stops past the main train station and get off at the Hauptwache, which puts you in the city center.