Air bed

Décathlon sells that stuff. Don't forget to buy the pump as well.
There is a Decathlon near Riquier station, and a small Décathlon city near the main station but that's less likely to have it.

Perfect, thank you!

absolutely agree with balso about Decathlon. I have one in my closet for when the waterbed gets a leak or when my brother pops in.

You'll need a foot pump. The supervalve inflates very quickly, but you may find that it boosts your luggage weight some. The smaller one is 3kg and the large very nearly 5kg

https://www.decathlon.co.uk/p/air-seconds-2-person-inflatable-mattress/_/R-p-107567?mc=8356245

or half the size and just slightly cheaper: https://www.decathlon.co.uk/p/air-seconds-1-person-inflatable-mattress/_/R-p-108836?mc=8356244

These are the UK website, the same stock in all the stores across Europe... inexpensive and durable and decent quality... there are stores in most towns.

OH Decathlon! You'll end up finding a ton of stuff you didn't know you couldn't live without. Love that store.