You might want to consider Alaska. They offer 500+1000(myalaskair sign up) miles on enrollment. NW flights are elite qualifying, CO flights (even on NW flight numbers are not). You can also earn miles in AA and other airlines so this is a nice program for someone who flies infrequently on a range of different airlines.
They serve both DC airports and charge only 20K miles for a domestic award (incl alaska) so it is possible to take a trip from Washington to Alaska for only 20K miles - maybe something to aim for.