For brass instruments:

Brass Player's Christmas Survival Kit >
45 Christmas Carol arrangements for brass quintet and larger ensembles

20 Chistmas Carols for Trumpet Duet >
Fun arrangements for buskers, or pupil & teacher duets

For piano / vocals:

25 Christmas Carols for Children (piano songbook) >
25 Christmas Carol arrangements for easy piano. Includes full lyrics for each carol - perfect for family sing-alongs!