I am a cheap flier - I book the cheapest seat and under-seat baggage and Ive never needed to find launderettes or anything
I have to admit Ryanair is a challenge but the rest I usually have space for souvenirs coming home
Wear your heaviest bulkiest clothing in the flight - so trainers/hiking shoes/big coat/heavy jumper/jeans ( obviously depends on when and where ) I also take a cross body bag - big enough to carry passports, wallet, phone, vape - but small enough its not spotted at the gate ( Ryanair )
If your case has a zipped lining - use it to tuck a pair of flats and sandals into the corners of the case
Don't forget to roll socks and pants and fill the shoe space
Forget soap bag. I do an Ikea food bag with liquids - just cos here we can leave them in, not always the case abroad. I do another with non liquid toiletries, another with plugs and cables. Putting into bags, you can squeeze them into the tiniest of spots where as a soap bag takes a lot of room
Roll your clothes. Im not a teeny person, but in Ryanair under-seat luggage I can easily fit in a clean pair of jeans, a frock, a lightweight jacket, 3/4 teeshirts , couple of jumpers. I rely on the accommodation having a hair dryer and I forget about straighteners
I can do two nights with just a vanity case - even the easyJet flight attendant questioned me on that one :)