Stopover rules vary by airline. For domestic rewards, AA's rules are the most liberal by far. They allow one stopover AND one open jaw. Moreover, as long as you stay under the MPM, your stopover can be anywhere (not just DFW, ORD, etc) and you can stay for nearly a year (well, the whole ticket is valid for a year). So, for example, you could do something like this:LAX-DFW-MSY<stopover>-MIA<open jaw>NYC-LAX.

Other airlines that I know allow stopovers on domestic award tickets are DL and AS, but on AS you're pretty much locked into PDX and SEA as stopover cities. Furthermore, on both of these airlines, if you opt to stopover, then you can't take an open jaw as well.

The other carriers I fly (UA, NW, CO, US) permit open jaws but do not allow stopovers on domestic award itineraries.