Wear one outfit, pack for 3 other days, wash tops and underwear twice instead of just the once. I cannot bring myself to care about trying to wear different outfits on different days. Who's counting? Just rewear.
It sounds like you are going somewhere hot. If you favour long dresses, maybe travel in one, packing a light pair of trousers and a pair of shorts. The default is to travel in jeans but all too often I find I only wear them on the plane, so better not to bring them at all. I also always overpack socks - if you're going to live in sandals most of the week, you don't need many socks.
My daughter is a supreme light packer, so many of her hot weather clothes are little dresses and vest tops that fold up to nothing at all. Oh to be 18.
I don't see the point of buying toiletries out there either, but hand washing is no big deal and only takes what, 10 mins max? It seems so much easier to me than lugging 10 days worth of clothes around. It's different if you are travelling with kids and having to find multiple clean outfits every day, but for one person washing a top, vest top, pair of knickers and a bra is trivial.
The ryanair bag really is tiny though. As PP said, think of it as a very short holiday on repeat.