I'm the same as you; picking outfits is about the extent of my back-to-school preparation too! I find that having a simple routine for the first day helps settle in an otherwise overwhelming transition. It's also the one time of the year where I don't mind getting up extra early so my kid can sleep in a little and start the day less frazzled. We take our time, eat breakfast together while chatting about anything exciting that might be happening at school, and pick a special snack for recess-- nothing elaborate, but it helps build anticipation for the first day!
I've also heard of families throwing confetti or having a little dance party when their kids put on their new uniforms, which I think is adorable, albeit extra. We could probably do with adding a bit more pizzazz to our mornings; they're usually pretty subdued.