In Mainz (near Frankfurt), the Hotel Kurmainz has a very nice restaurant.
Frankfurt, Germany
Restaurants in Germany, help wanted
In Germany you pretty much can't go wrong with local cuisine. I think the Polizie close down any place that's not up to snuff. The best German food in Darmstadt is probably the 'Sitte' (pronounced 'zit-teh') on the corner of Hugel Strasse and Karlstasse. I've also been told the Darmstadter Brauerei is tops, though I never went there myself. There's a great Greek place on the west side of Heidelberger Strasse, near Goethe Strasse. I think it's called the Thesselonika. Beware, though, there are a couple of mediocer Greek places in the same area. This one has white curtains and a pair of Greek columns just inside the entrance. There are also a number of good places in Eberstadt, a nice old town to the south which has been swallowed up by Darmstadt. Park your car near where Heidelberger Strasse becomes a one-way street and pick any place in the old town. The Heidelberg Hotel, a little south of downtown Eberstadt, has a very nice Italian restaurant. Beware, though, that driving back from Eberstadt can be a nightmare. Too many one-way streets. As for finding a good Mexican place, I'm guessing this is your first trip to Europe. Stay away from any place that calls itself Mexican over there. Have a good time.