how should one cool down after climbing 900 steps

You really did that. great then now you should do some Breathe in and Breathe out exercise. stand where you get fresh air. Drink plenty of water. relax your legs a bit as it may hurt towards the end of the day as its not simple. you have walked 900 steps which is tremendous.

Stretch ur legs and back, Take deep breath, close ur eyes and relax.

Better if u do not drink water immediately. Drink after a gap of 10 mins.

stretch yourself,sit or sleep on the ground for a moment and drink little amount of water.

I'd do stretches, go for a light jog or a brisk walk, and then stretch again.

A quart of Miller Highlife works for me.

l do a staged cool down - slow ur rate of climbing until u r almost stopped & then start walking. l used to walk down a bunch of stairs after climbing them, but l would trip when l was really tired & it is just as good to walk. Just keep the muscles moving for a while - sitting down right away causes extra soreness for me.

l agree that u should not start drinking right away, but u should try to hydrate a little extra several hours before exercising this hard - it seems to help me.

Good Luck