Day Trips from Frankfurt

There are lots of day trips you can make from Frankfurt, though I hope you will plan on spending at least one day in the city as there is lots to see here, everything from medieval churches, Jewish cultural sites, farmers markets, applewine gardens and lots of museums.
The Museum Riverbank Fest is at the end of Aug. and needs close to 2 days to see everything. This is one of the largest cultural fests in Europe.

Towns to visit - Limburg, Marburg, Idstein, Eltville, Seligenstadt, Büdingen, Gelnhausen, Bad Homburg
The Hessen Park Open Air Museum is well worth a visit as is the Saalburg.

These are events happening in the Rhine Main region from middle to end of Aug.
* 12-17 Aug. Bernermer Kerb, Frankfurt Bornheim
* 12-21 Aug. Applewine Fest, Frankfurt Ross Markt
* 12-21 Aug. Rheingau Wine Week, Wiesbaden
* 12-21 Aug. Wine Fest, Neu Isenburg
* 19-21 Aug. Elvis Weekend, Friedberg
* 19-22 Aug. Wine Fest, Rüdesheim
* 20 Aug. Night of Candle Lights, Dietzenbach
* 20 Aug. Summer Nights Fest, Bad Soden
* 20-21 Aug. Arts & Crafts Market, Darmstadt
* 21 Aug. Essbahnstation International Street Food, Fechenheim Mainkur
* 25-28 Aug. Mainzer Wine Markt, Mainz Stadt Park
* 26-28 Aug. Museums River Bank Festival, Frankfurt
* 27 Aug. Rhine on Skates, Rüdesheim
* 31 Aug.-9 Sept. Rhinegauer Wine Fest, Frankfurt Fressgasse

Yes, Heidelberg is a good choice.

Thank you, Ms.Jo, for your suggestions. We do plan to spend 2 days in Frankfurt and to go to the Museumsufer Fest. So much to see and do and so little time.

"We have planned a K-D cruise of the middle Rhine one day and are looking for suggestions for other day trips. "

It takes just 1.5 hours to see the best Middle Rhine scenery by boat. With a day, visiting the towns and castles that dot the river will enhance your introduction to Germany. If you'd like suggestions for this day as well, feel free to share your time parameters and interests.

Not sure why you think you need to leave Frankfurt to see something. I'd consider the Palmengarten or Bergerstrasse instead. I plan on taking one of these walking tours later this summer.

I think a cruise is an excellent idea. But as the others already mentioned, why leave Frankfurt and spend hours on the train ??? The region and Frankfurt itself have so much to offer.
Maybe it is good idea to just hop on a train for an evening trip and check out some of the smaller towns/ cities/ sights surrounding Frankfurt ( less than 1 hour ride ) like Bad Homburg, Wiesbaden, Darmstadt, the roman fortess Saalburg, the castle of Weilburg etc.

I suggest traveling to Fulda, a beautiful town worth visiting. Its an easy ride by train as well. https://www.youtube.com/watch?v=VtUmwIQM1AY

To spend some time in historic Sachsenhausen would be high on my list. If is south of the river, and easy to reach by foot via the bridges. You can also check in with local travel agents (located on every block, it seems) and inquire about bus trips around the area. When we went we were disappointed in the zoo, since we have larger ones in the US and so this one was not a big deal.

Sachsenhausen is a bit full of local and tourist drunks. Hoechst is just as historically atmospheric, and you can get there with a normal Frankfurt central transport ticket by S-Bahn or tram.

I would suggest Wiesbaden, Heidelberg, Koblenz , from there Bad Ems and Kaub am Rhein, or Karlsruhe, numerous choices are available ...depends on far you are willing to travel from Frankfurt and your specific interest area.