On AA domestic award tickets you can have a stopover of any length in any connecting city, in addition to your roundtrip ticket, at no charge. The stopover has to be arranged over the phone, but even that service charge can be avoided by originally holding your roundtrip itinerary online and then immediately calling in to make the necessary changes to include the stopover. Then it stays an online generated reservation that can be changed during the hold period over the phone but is completed online for just the online booking fee. And of course u can change dates and times on any segment after it is ticketed without any charge.
United, on the other hand, does not allow free stopovers on domestic award tickets, giving AA a competitive advantage that in my case makes it my first choice airline.