Anyone with some ideas of what to do on the 18 hr. layover in Frankfurth?

The Shereton Hotel at the airport offers day rooms. You could catch some sleep, get a shower & a decent meal. On your way to Namibia? Make sure your bags are checked through for easiest transition. There really isn't much aroud the airport. You could catch a train into Frankfurt, they run like every 20min.

Yes on way to Nanibia, in several years. I Hope.

We caught a cab to the zoo.

Underneath the airport is a shopping area and the train station. Wife and I spent time shopping and having a lunch. We also bought a city tour ('Grey Line'?) in a bus that took us around for a couple hours. Time well spent. Back then (it's been six years) we could stow our luggage in a short term storage area they maintain there.

On a previous trip my buddy and I took a train into the city proper and found a bar open during the day (not many of those). Dispite a language barrier we had a very good time drinking with a couple locals. The more we all drank the better we seemed to understand one another. Took a cab back to the airport and just barely got our butts on the Air Namibia flight to Windhoek.

We took showers and ate snacks at the Delta Crowne room. We took the train downtown, had lunch and visited the zoo. The Crowne room is very nice and has Internet access, free drinks and light food. We took others advice and intentionally stayed awake until we boarded the flight to Windhoek. We arrived fresh and with very little jet lag.

Thanks for the Ideas. Hope to be able to use them.