If it's the cherry blossoms you want, I suggest taking the Blue line metro to Arlington Cemetery (still on the Virginia side of the river) and then walk across Memorial Bridge towards the Lincoln Memorial. You'll have a great view from the river crossing and once on the other side a nice walk down the river to the Tidal Basin and the Jefferson Memorial. Note that it is the Jefferson Memorial that is near the 14th Street Bridge and not the Lincoln Memorial.
For the return, you can decide if you want to:
-return to DCA the same way
-walk back to DCA via the 14th Street bridge and the Mt Vernon trail
-cut over to L'Enfant Plaza and catch the Yellow Line metro back to DCA.
http://www.wmata.com/rail/maps/map.cfm