I know exactly the kind of sled you are talking about. If you wanted to you could still paint a single snowman, poinsettia, or a Christmas tree broken up across the front. Full bodied designs like those do look OK across a slat surface. It's just a matter of the look you want.
If you don't want to do that, you can paint each slat with a different motif or all of them with the same - say little trees down one, little snowmen down another, etc.

If I were painting motifs on the slats I would probably put little snowmen with snowflakes in between each one on a blue background, trees with Christmas balls between each one on a red background, or poinsettias with berries and/or holly leaves between each on a white background and in this case probably paint some edging lines in metallic gold around the sled, on the runners and maybe even on the poinsettias themselves. You could also paint all of the slats with a running holly & berry design which would look very nice.

These are just a few ideas maybe someone else here will come up with some more.

How about painting a background, navy with stars and then a snowman on each slat.

Great suggestions.......Thanks so much.....I'm inspired!

Just another suggestion.....you could also take a picture from a magazine, book, large Christmas card, etc. break it up and decoupage it across the front. Makes for an interesting piece.