I always travel with a small underseat bag only, sometimes for up to 10 days. As do my kids. I do this for summer and winter trips. Summer ones are easier for obvious reasons.
My advice is to pack something like the following (i am currently on a 7 day holiday with two teenagers)
1 x denim shorts
2 x midi length, lightweight skirts in block colours (think satin- like fabric that can be rolled, my skirts are from Hush)
1 x dress (this year it’s a loose camo one that hits mid calf but previous years I have had more fitted ones to dress up or down)
3 x vest tops in same lightweight fabric and block colours as skirts therefore can be mixed and matched with skirts or shorts.
1 x black vest to sleep in - I sleep
in this at home and could at a push wear it as clothing if need be as it isn’t specifically a PJ vest.
3 T-shirts
1 x Fit Flop flip flops in black - these are actually very important as will go with most outfits, and provide support akin to trainers for walking etc. I never skimp on buying good supportive flip flops now. Archies are similar.
1 x lightweight tan sandals that pack really flat.
If I take trainers I specifically take a pair of black Allbirds as they are super light, and comfortable and can be washed if needed be, they also squish right down. Not the most beautiful, but I dont wear them for style - I stick to sandals etc if dressing up more. I often wear these on the plane.
1 x swimsuit /bikini
Toiletries all in miniatures and therefore fits in bag easily. I take face creams/makeup/shampoo/hair products but buy sunscreen/body moisturiser/deodorant airside in miniatures that I use up whilst away. Cream deodorant like Akt in a mini pot is also a good alternative.
Collect perfume miniatures over the course of the year that I take on holiday or decant usual perfume into mini atomiser.
phone/chargers/applewatch
1 small pouch for jewellery
HRT because you’d have to wrench that from my cold dead hands.
I wear lightweight trousers to fly such as ‘dressier’ joggers or black looser fit trousers, and layer a vest or T-shirt with an oversized button up shirt in white which can be dressed up or down and provides warmth if needed. Sometimes I take a pashmina instead for the same reason.
This all fits in one rucksack / tote bag by rolling and clever folding, and I nearly always still have something I didn’t wear on my return.
Enjoy your trip!
ETA: forgot to add very small cross body bag for essentials only on days out/evenings.