In general, not RE just to stormkitten.
Re: Metro.
I'd suggest another station than Foggy Bottom.
Arlington Cemetary (Blue line).
It's closer, just a short walk over the Memorial Bridge to the Lincoln.
See http://www.wmata.com/metrorail/systemmap.cfm
The Metro map does not show the bridge, but it is there :) It is closer than walking from Foggy Bottom, and *alot* more scenic. You can check out the Cemetary. You can check out the Potomac and the monuments across it as you cross (the bridge crosses from Virginia into DC). Walking down 23rd Street from Foggy Bottom; boring and slightly longer.
Also. If you have any way to do so. Bike in from the closer suburbs. It's a nice ride, and no messy Metro or crowds.
http://www.waba.org/new/paths/mtver.php
Lots of trails hit the Mt. Vernon trail. And once on the Mt. Vernon trail just follow your nose over Memorial Bridge to the Linclon Mem.
I would not factor the Cherry Blossom. I don't think the parade is until later?
http://www.nationalcherryblossomfestival.org
No major activity on that Sunday, recent cold weather has pushed back the blossoms anyway. Tho you may get a peek this Sunday, it's not the real deal.