The two songs I've endeavored to learn are "Time in a Bottle" by Mr. Croce, and "Blackbird" by the Beatles. I really enjoy playing these two songs. They sound great on my D-03, and stretch my fingers out nicely. They are far easier to play then they sound. Years ago I would have avoided a song like "Time in a Bottle", but now I know you have to challenge yourself with new songs. Break them into pieces, and then string those pieces together. Practice them until you despise them, then play them some more.
Someday I aim to be able to play this song with relative ease. Yes, I'm aiming high, but why not give it a shot?