Frankfurt Trip Itinerary ideas

Who told them Frankfurt only needed half a day! (where would shopping play a part in sightseeing?)
The city is chock full of interesting sites, but it depends on what they are interested in seeing.
Dozens of museums for a wide variety of interests, there is a large amount of Jewish cultural heritage sites, about 4 medieval churches including one of the oldest churches in Germany, the Carmelite cloister with the largest religious wall paintings north of the Alps, the church where the Holy Roman Emperors were elected since the 1300s and crowned since the late 1500s, lots of great Farmers Markets, the Klein Markt Halle, the Botanical Gardens, huge parks, beautiful turn of the century neighborhoods. Head to the neighborhood of Höchst which is part of the Half-Timbered Route in Germany, as they have about 400 half-timbered houses as well as a lovely little Schloss and a palace.

They should not do Strassburg as a day trip unless they enjoy spending hours on a bus. These types of day tours are advertised heavily on Trip Advisor with no thought given as to how long it takes to get there and back. Stay away from the tours offered by Viator too. This is a company that sells other peoples tours.

For the Rhine, I would look at Tours By Locals if they need to have a tour, though this is a trip that is very easy to do on your own.

"...their short trip to Frankfurt."

Does this mean only that they are flying into FRA airport? Or that they have booked 4 nights in Frankfurt proper?

"Things that were of interest to them were potential trip to Stratsbourg France as well as Rhine river cruises."

If they don't plan to spend much time in Frankfurt, and if they have hotel flexibility... their wishes can be granted. it's a simple matter to take the train from FRA airport to Strasbourg. Frankfurt to Strasbourg takes about 2 hours if you take a direct train.

So maybe on Day 1 they head into Frankfurt proper for sightseeing and one night. The next morning they take off for Strasbourg for 2 nights. That's a nice amount of time for Strasbourg. On the 4th day, they catch a train to the Rhine for a cruise and a castle tour, then a train back to Frankfurt for their final night. How does that sound?

(On Day 4, your parents' luggage could be stored in a Mainz station locker just prior to the cruise and retrieved on the way back to Frankfurt. Mainz is on the way to the cruise, and on the way back to Frankfurt as well.)

As Jo points out, Frankfurt is not a good base town for a day visit to Strasbourg.

When is this trip, anyway?

For a 4 day itinerary:

Day 1 Frankfurt - Museumsufer, Frankfurt River Cruise

Day 2 Frankfurt - Day trip to Strasbourg , Baden - Baden, or Rothenburg. Choose one location

Day 3 Frankfurt - Römerberg, Palmengarten, St Paul’s Church, Kleinmarkthalle

Day 4 Frankfurt - Fly Home

https://www.timeout.com/frankfurt/things-to-do/kleinmarkthalle
https://www.getyourguide.com/frankfurt-l21/1-hour-panorama-trip-in-frankfurtmain-t2951/

Rothenburg is not a day trip from Frankfurt. They would spend between 6-8 hours on the bus.
Why go all the way to Baden-Baden? There are spas and baths all over the place near Frankfurt - Mainz, Wiesbaden, Bad Homburg, Bad Soden, Bad Nauheim.
If a medieval town is something they want to see, they could visit Büdingen, Gelnhausen, Marburg, or Limburg. Those are all towns that are an hour from Frankfurt by train and they also were not bombed in the war.
Interesting sites near Frankfurt would be the Saalburg Roman Fort near Bad Homburg and Hessen Park Open Air Museum. Bad Homburg itself is very pretty.
On the Rhine, visit Eltville and Eberbach Monastery. Spend a day visiting Mainz. Visit Rüdesheim, cruise to St. Goar.

The river cruise in Frankfurt leaves a lot to be desired. There are so many other things you could do or see.

Tell me the dates and I will tell you events and festivals are happening in this area.

Thank you all for your helpful replies. My parents will be in Frankfurt for 4 days but have not booked hotels yet as I am trying to help them with their itinerary. They will fly to Frankfurt and have 4 days to explore flying out to Moldova after for 2 weeks. Trip will take place in early September

Agree with R.Jean and Ashley.