I have an idea of what to bring, but would love to hear some brilliant suggestions!

I'm doing a ride on Saturday that's going to be 27 degrees when we start, up to 58-60 degrees during the day, and potentially back to the 30's/40's when I finish. It's a brevet, so no SAG or drop-offs, and it's a loop, so can't stash stuff to pick up later.

I have a slightly oversized seatpost bag, but that will be filled with drink powders - everything needs to be carried on me, wrapped around the seatpost or hung off of aerobars.

What would you wear?