What to do in washington DC, on a metro line? halloween, fall foliage, thanksgiving, touristy, etc?

The Air and Space Museum should have been the first one you went to. Do it and all of the rest. For Halloween go to Georgetown. Get or rent a bike and ride down the bike trail past NAtional Airport, and you can go all the way to Mt Vernon on it. Or ride up the C&O C bike path. I think you may be able to rent a bike in Goergetown near the river.