Thanks a lot Paint Horse & EmailKid for your suggestions.
In the end my DFW layover turned out to be quite different than what I had originally planned...
I arrived at DFW at 8.30AM from Hawaii and my onward LH flight to FRA left only at 4PM (separate ticket). What I hadn't thought of is the fact that the LH check-in desk only opens at 12.30PM which meant I was stuck with 4 pieces of luggage and therefore my plans of going either to Dallas downtown or to Grapevine were no longer an option. I ended up booking a room for a couple of hours at the DFW Grand Hyatt which is very conveniently located (2 minutes from the LH counter). Although I was quite tired from the red eye, I decided to spend my time at the gym and in the outdoor pool which has a great views of the different runways. Airplane geek heaven with a cool 32 degrees celsius at 10AM!