So, lots of options here. I'm guessing that since you're on FT you have some interest in miles/status, and therefore are either using miles for this or otherwise have some thoughts about airline to use... which will make a big difference.

If not, other thought would be - does she want to actually see a bit of the city, or just chill at an airport/nearly hotel? I know that I'd like to get out, get some air and get away from the airport environment, but maybe she'd rather stay put. And a related comment would be on getting local currency, a necessity for getting out and about but not needed if just staying at the airport.

So thoughts below are based on (my experience of) airport itself, ease of getting to/from city, costs. I guess that there will be airport hotels near each one with the usual range of luxury and cost, so won't mention that.

In no particular order:

London. Heathrow is okay as an airport. Getting into London is quick and expensive (HEX) or cheap and slow (tube). Costs not too high.
Amsterdam. Airport easy, and transit to city cheap and quick. Costs pretty reasonable, and quite a fun place (!)
Paris. I find CDG a mess, and transport into Paris is painful. And it's expensive. But it's Paris.
Zurich. Airport and city transfer brilliant, but everything is nose-bleed pricey.
Frankfurt. Airport is fine, but I have never actually been into the city.... no clue. Likewise Munich. Sorry.
Madrid. Airport a bit mazey but okay, metro into city easy, fun place but imo less easy for non-local speakers than some of the others.
Warsaw. Airport a building site last time I was there, but that was a while ago so maybe very nice now. Transfer pretty easy and everything is cheap.
Rome. Don't do it.

I'd suggest AMS. Or Madrid. Or London.