Harmonize any lutheran chorale. You can make a Bach-style harmonization or use 20th century harmony. If you are exceptionally bored, you can write a full fledged chorale prelude or chorale variations. Use any instrument(s), as long as the music is based on the chorale melody.
For those without easy access to the original melodies, try harmonizing this short German Chorale melody, Das Alte Jahr vergangen ist.
Attached is the unadorned melody in MIDI.
My slightly ornamented organ harmonization
Have fun.
(I might come back later with a partita on this chorale; it's a very nice melody.)