Town near FRA

If your flight to Seattle is at 09:00, you should really plan on being there 3 hours before.

Unfortunately, when a trade show is in town, not only are the hotels in Frankfurt expensive and full, but all of the near-by towns are too, like Mainz, Wiesbaden, Bad Homburg, or Darmstadt. If you go to far away, it will be harder to get to the airport. What are the dates of your visit? The Tourist info often has blocks of rooms that are reasonably priced during the trade shows.

Give this place a try, as I think they keep their prices the same during a trade show, though they may already be booked. It is in Niederad, which is just 2 stops on the S-bahn from the airport.
http://www.kleine-villa-frankfurt.de/ueber_uns.html

We stayed in Aschaffenburg before our flight out of FRA. The train was simple, running right into the airport, ride the escalator up, and be at check-in.

Thanks. The flight is at 11 am on 13 September

We ran into the same problem in a few years ago. We didn't turn in our car the night before but ended up staying at the Rheinischer Hof Hotel in Ginsheim which is about a 20 minute drive to the Frankfurt Airport. They have a good restaurant on premises and it is located on a small arm of the Rhein River. We have subsequently stayed there on several occasions and always enjoy our quiet evening on the river.

http://www.schaefer-hotel.de/

"Does anyone have a recommendation for a small town near Frankfurt where we could drop off our car the night before, have one last good German meal and beer, and then train to the airport in the morning... The flight is at 11 am on 13 September."

Did you check around? It's worth checking the towns Jo has suggested - don't dismiss them as options just because they're within a certain radius of Frankfurt. I'm very fond of Mainz, about 20 minutes by direct train from FRA. It's got a huge pedestrians-only zone with old-world buildings, shopping, sights. I just checked that ultra-exotic booking engine Booking.com for Sept. 12 and found a double at the Königshof near Mainz' main station for $86. Even the Hilton Mainz goes for $126 on that date. I like to get to the airport early. No problem with that from Mainz on the 13th; leave Mainz at 7:43 and arrive 8:05 at FRA.

Heppenheim, Marburg, Bad Nauheim, Idstein, Büdingen, Seligenstadt, Bensheim, or Wetzlar. All are close enough that you could reasonably get to the airport in time, but small enough that they may escape the notice of conventioneers. And all are very attractive.

The website for Königshof in Mainz shows the cost of a double room at 85€. When I was last there in 2008, there was an Italian restaurant and a Chinese restaurant nearby, but no German restaurant. I didn't feel like either of these, finally settled for a storefront döner kabab shop nearby. I think there are now a few more restaurant options in the area. Other than that, it was a perfectly acceptable hotel, convenient, comfortable, great breakfast, free wifi, and the price was good. It's right across the Bahnhofsplatz from the train station.

Mannheim and Karlsruhe are on the high-speed ICE line to Frankfurt airport, about ½ and 1 hour respectively. Heidelberg is about 15 minutes from Mannheim by S-Bahn. Some years ago I stayed in the Black Forest spa town of Bad Herrenalb the night before flying out of FRA in early afternoon. Including the 38 minute streetcar ride into Karlsruhe, it was about 2 hours from Bad Herrenalb to Frankfurt airport.

Like Carla above, we stayed in Aschaffenburg before a flight out (and a couple times on the way into Germany). It is about a 1 hour train ride on an IC or an ICE. We stayed at the Cafe Fischer. The Theater Krause and Uwe's Bistro have been two restaurants where we enjoyed German meals. Pizzeria Pomodoro e Basilico is a decent Italian restaurant. As Aschaffenburg is a theater town, there is a wealth of restaurants and services. The shopping district is nice and we enjoyed the museum in the Residenz.

We stayed in Heppenheim and loved it. It was very convenient to the airport and had a quaint old centre.