AA runs only 2x daily DCA-YYZ and the fall schedule shows both being CR7s so you'll most likely be departing out of the D or E gates. That's a 3:00 pm departure and a 5.5 hour layover means you have to be on the 5:30 am out of DFW which gets into DCA at 9:30 am. That's an early departure! The good news is that 9:30 is moving past the morning rush surge in DC. You should have at least 3.5 hours of time to go see stuff inside DC - one to two museums are doable. Bigger museums like the National Gallery can eat up all of 3 hours.