For me:
fill my large pots with water before the pressure fails (I am on floor 11 but there are 24 floors so I’ll have some for a while).
Go to the basement -2, collect my 1 ring camping stove to cook on my balcony (or outside in the main square). And my hardsided cooler (to act as regular fridge). And my bag of emergency cat litter.
Put my solar panel up to recharge my power bank if it’s not absolutely full.
clear the walkway between the front door, kitchen, dining table and on to my room/bed.
Pull out the candles and matches for later, putting candles into their proper holders.
Charge up my rechargeable lantern from the power bank if it’s not full.
Go to the shop nearest me and buy 2 bags of ice. Use them in the cooler, and put in the things I am always opening the fridge for - milk, jam, cheese - and a dinner for tonight , and take a dinner from the freezer for tomorrow to defrost (keeps cooler chilled but also means I don’t touch freezer again for 36 hours so it might stay frozen).
Set up alternative toilet facilities (probably 1 toilet ok for wee, I’d have 2 flushes available in the tanks for the first day or so, but then builders sacks and kitty litter in a bucket (or dry toilet bowl) would be the solution. Unless I managed a 3rd flush from my watering can. That aspect was much easier with a rain butt in the garden of my house and a bucket/watering can to transfer the water.
Then grab my cosy blankets and cushions, open wine, and my emergency chocolate stash, grab a book…as once I’m sorted, I may as well relax and enjoy. (No DCs here, and “only” an adult DC and DH at home when I’m there so much easier now).