My pops and I are planning a trip to DC for late September. We're in the process of setting up a tour of the White House and Congress now.  Most of the attractions in the area of the Washington Monument appear to be within walking distance.

So I'm thinking about seeing the WH and most of the monuments day 1, Congress, National Archives and the Supreme Court on day 2 then day  3 at Air and Space and whatever comes up with what free time we have left.

We only have three days, but is there anything we should absolutely see while we are there? I would really like to sneak over to Arlington to the Tomb of the Unknown Soldier.  Open to suggestions and thanks.

Will you have a vehicle or dependent on feet/taxis?

I always try to visit Arlington every time I'm there but it will realistically eat half a day.  Metro is usually a great option for touring the city but they have been having some maintenance problems the last few months.  check availability but if it is up and running then get a full day pass and hop on and off as needed.   It's pretty easy to navigate and user friendly.  Day one sounds pretty ambitious but if you don't try to see everything in each location you may be able to make it to all those places.  I have visited DC over 20 times for work so here are some of my favorites

International Spy museum - not free but worth the money
Off site air and space museum.  the main museum on the national mall is good but the off site location has much more to see.  It's outside the city so takes some travel but worth it
There used to be a restaurant named Marrakesh that was really good.  It's about a 3 hour meal and kind of dinner show so don't be in a hurry.  They were closed last time I was there and I'm not sure if they have opened back up or not but if it is open it's worth eating there.  It was cash only but there is an ATM in the front lobby.
holocaust museum - very humbling and worth the trip
Postal museum has always been a favorite of mine.  lots of things you wouldn't expect to see there
Natural history museum is great but takes about a half day to do it all
National history museum is worth adding to the list.

I'm not sure where you are staying but I have had great luck staying at the Marriott residence inn in college park.  It's outside the city so much cheaper but right on the metro line (red line I think) so you can be in the city in 15 ish minutes.

hope that all helps.  I'm not a big city guy at all but I love to visit DC.  the best guide book I found was the DC for idiots book.  have a blast and accept that you will not see a fraction of everything in just 3 days.  If you have any questions post them and I'll keep an eye out.  Have fun.

ooh and see if you can get tickets for the Washington monument.  Great history lesson given on the way up and amazing views from the top.