The Hilton Garden Inn DFW Airport South has a day rate which I know is good from 7am to 4pm since that's when we need it. It also offers a free shuttle to and from the airport.

You could buy an AA lounge pass for the day and store your luggage there. I don't know if they have showers in the DFW lounge but the lounge itself is very nice and comfortable for napping or relaxing between flights.

Wildblueyonder, I hope you aren't too tired from your flight to enjoy the city.

First of all, Dallas is one of the 5 or 6 largest metros in North America, with a metro population between 6 and 7 million people. It has many things to see and do, but I realize you'll be tired from your hours in the air.

Downtown Dallas is probably the best "bang for your buck" since it conntains many attractions within a small area. You will want to see the JFK museum there, since your friends back in the UK will undoubtedly The museum is easily reachable in downtown Dallas, but there are other things to see that are doable in a few hours:

. Go to the top of Reunion tower for a stratospheric view of the city
. Visit the original Neiman-Marcus department store, which is an icon of Texas retailing
. There is a huge sculpture garden in Pioneer Plaza containing an entire wild west cattle drive... dozens of life-size cattle, 3 life size cowboys. This is an excellent photo opportunity to show your friends at home.
. The Dallas World Aquarium is unique among the many cities I've visited. It is actually an indoor zoo in downtown Dallas, just 2 or 3 blocks from the JFK museum.
. The McKinney Avenue Trolley is a line of old trams over 100 years old that travel through one of Dallas' most charming and urbane districts, to a place called West Village.
. Downtown Dallas is filled with many examples of beautiful sculpture. The architecture of the downtown is interesting... the so-called Arts District has many examples by some of the best contemporary architects in the world.
. Downtown Dallas has many museums in addition to the JFK museum, including 3 art museums, a Holocaust museum, a museum of local history, and others.

I hope you won't be too fatigued.