There are still a lot of DC-9 flights left. You probably should figure out where you actually want to go and visit NWA.com and check flight status between those city pairs. That will show you the type of aircraft operating on those routes.

This link will show you the current DC-9 flying in the US at any given time:

http://flightaware.com/live/aircrafttype/DC93
http://flightaware.com/live/aircrafttype/DC94
http://flightaware.com/live/aircrafttype/DC95

Other aircraft:

737-200- Aloha Airlines in Hawaii operates a lot of these between the islands.
DC-10, I am not aware of any scheduled carrier still operating these. NW and (soon) ATA will operate them on a charter basis
L-1011, again ATA is the only airline I am aware of that operates these and only on a charter basis. (Once last December, however, I was flying ATA and they did an equipment sub from a 757 to an L-1011, talk about being lucky!)
727- Check out Champion Air, I know they operate a few of these (I see them parked at the Berry Terminal at DTW every once in awhile!)