Layover as in overnight? Where is the hotel? There should not be a lot of traffic mid-day but if there is inclement weather or a morning accident, there could be congestion - 2.5 hours pre-flight should be more than enough time regardless. Afternoon rush hour start around 3:30pm so you would avoid any of that.

If you have elite status or are sitting in a premium cabin, use the premium passenger security line. It can be a major timesaver.

BTW - for your monument walk, jefferson is the one that is off by itself - you might want to have the taxi just wait for you if you negotiate for a few extra bucks and catch it from your hotel or elsewhere in the city. You can walk lincoln, vietnam, washington monument and up to the capital (pass the smithsonian museums) or over to the white house. I don't know the visitor process at the capital these days, you'd definitely need tickets to go inside the white house if tours are even being allowed at the moment.