Hmm...famous classical tunes?
Greensleeves has always been one of my favorite melodies, and while pretty simple, I've heard lots of variations of the theme that add complexity if you wanted to spice it up, or even improv a little.

Or a few other relatively famous tunes:

Nutcracker March-Tchaikovsky
Blue Danube Waltz-Johann Strauss (always reminds me of matador Bugs Bunny :) )
Canon in D-Pachelbel
Starlight Waltz-Charles Brainard