Consider staying in Frankfurt, since it isn't on the tour, but offers many historic sites, great Farmers markets, several medieval churches including one of the oldest in Germany, lots of museums, and because it was the center of Jewish culture in Germany for many centuries there are a lot of Jewish heritage sites here.
If you prefer a Rhine town, then Eltville would be lovely. Promenade along the river with no trains running by, lots of half timbered houses, the near by Eberbach Monastery, lots of wineries, a small schloss tower and a dry moat filled with roses.
These are some of the events happening in Frankfurt and the near-by towns and cities in May.
* 4 May Riesling Day, Frankfurt Palmengarten
* 4-5 May Roman Market, Saalburg
* 4-5 May Plant Market, Hessenpark
* 5 May Frog Parade & Garden Market, Büdingen, 11:00
* 9-12 May Wine Days on the Rhine, Mainz Riverbank
* 11-18 May Green Sauce Festival, Frankfurt Hauptwache
* 11 May Night of the Museums, Frankfurt & Offenbach
* 25 May SaTOURday, Frankfurt Free Museum Day (not all of them)
* 22-31 May Fressgasse Fest, Frankfurt Fressgasse