Yes, you can use days from old WDW tickets, assuming they have not expired yet. And if you have a WDW hopper ticket, they will get you a DLR park hopper too.
For a day, I highly recommend getting to the parks as early as possible. If you are parking in the garage, get there even earlier because you will want to be one of the first cars in the lot and the garage doesn't even open until half an hour before the park, so if you aren't one of the first cars in there, you probably won't make it to the park by opening time.
If you are used to WDW, on a one day trip I would highly recommend these rides:
-Indiana Jones (you'll love it, especially if you like Dinosaur)
-Matterhorn Bobsleds
-Alice in Wonderland, Pinnochio and Mr. Toad (all Fantasyland dark rides that WDW doesn't have)
-Pirates (it's 4654x better at DLR than WDW)
-Roger Rabbit
-Space Mountain (much better at DLR)
-Buzz (same as Pirates and Space)
and at DCA:
-California Screamin (if you're into roller coasters)
-Grizzly River Run (blows Kali out of the water)
and Monster's Inc.: Mike and Sulley to the Rescue
If you still have time, try DLR's It's a Small World or Finding Nemo, although Nemo tends to have massive lines.