Overnight layover in Doha, on a flight to SE Asia.
Strongly recommend the Qatar Air layover promotion (it runs frequently) -- includes free visa and deeply discounted hotels (think $50/night at the downtown Westin) for up to 4 nights. (IIRC it also includes airport transportation.) Plus it's a 5-star airline, really is much better than the US domestic carriers. They also fly 1-stops to many destinations that otherwise require 2 stops, and are comparable in price.
2 nights is likely enough in Doha -- folks should see the Museum of Islamic Arts (daytime) and Souq Waqif (night), and a boat ride in the harbor. Not a lot more for tourists, and most probably.don't need to venture into the desert. Plus it's like Phoenix, only hot. And there's a lot of haze (which has an eerie effect at night, but you probably don't want prolonged exposure.) And it's under constant construction, as oil-rich states tend to be.
All that said: we weren't quite that lucky. Our outbound flight from PHL was delayed by about 15 hrs because the original plane from DOH to PHL had to return mid-flight due to an on-board incident. This screwed up the connecting flight, so the airline gave us the free layover (except the hotel was more like the Ramada than the Westin, but it was free). With only 10 hrs on the ground, we did see the Souq.
Below are pics of Souq Waqif, the Islamic Cultural Center, and the ever-present haze.