I think that Legend from DAL-IAD would be the most fun, and you can fly them on an inexpensive fare if you are advance-purchasing and staying a Saturday. www.legendairlines.com
Also, if you want to try the new Boeing 717, I would try TWA through STL to DCA. TWA flies the 717 and the DC9 between STL and DFW, so I would check on their web site (or elsewhere) to make sure that you are scheduled on a 717. I flew on TWA from NYC to IAH last month through STL to make sure that I could try out the 717, and it was an absolute blast! Clean, quiet, powerful (like a Porsche accelerating), fresh air...in short, fabulous!