Go for something like a quartet/quintet (brass, string, woodwind, whatever). Quite frankly, I prefer writing those to writing things with more parts, unless you're going for a grandiose epic feel. If you do a full orchestra, a lot of the time you won't really have more 'parts' going on because you need to have multiple groups playing the same thing so each part doesn't get lost in the noise, anyways.
God Rest Ye Merry Gentlemen, if you feel like doing a more traditional Christmas song.