Recommendations for a small foldable rucksack please

[OMGitsnotgood]

When i travel with hand luggage only, i like to maximise the allowance with a rucksack which fills the underseat storage. However, that is too big for days out and the small rucksack I’d put in my case if taking more luggage is too bulky when travelling light.
I’d like a small, lightweight, foldable rucksack for water, suncream, tissues, wipes etc so doesn’t need to be too big. I know I can google but want to make sure it’s reasonable quality and strong enough for the items I’ve described.
Does anyone have anything like this and if so can you find a link please?
thanks

I've got one of these. I think it ticks all your boxes!

https://www.osprey.com/gb/osprey-ultralight-stuff-pack-s23?size=One+Size&colour=Tundra+Green

It does,thank you. Probably more than I was expecting to pay but I do want quality so maybe

I have one like this from decathlon which has lasted years so far. https://www.decathlon.co.uk/p/10-l-hiking-backpack-arpenaz-100-flower/344147/c4c311m897138
Its flat when unfilled and fits in my bag easily. I use it for water, suncream and waterproof jacket on holiday / festivals so pretty much the same as what you want it for.

There's a folding one too, even cheaper.