My tip - you can wear things more than once without washing them. There seems to be a generally held belief these days that if an item has been worn (even for a couple of hours) then it's dirty and can't be worn again. Not true.
This. Underwear isn't just for decoration, it's so that you can re-wear your trousers. Assuming you are sightseeing rather than trekking sweatily through muddy trails, a pair of jeans should be good for a week. A pair of light trousers that pack down small and you can wear those while the jeans are washed and vice versa.
Undies are necessary. Three t-shirts is loads. The universe doesn't melt if you're seen in the same outfit twice.
No need for a glam dress if you take a glam t-shirt to dress up a pair of jeans - this also means you don't have to take glam shoes.
Wear coat & jumper.
Toiletries - toothbrush, toothpaste, shampoo/conditioner, shower gel, mini hairbrush. The fifteen types of cleanser, toner, body scrub, hairwash, facewash, bodywash, elbow-wash etc smell lovely but they have been sold to you by cynical manufacturers wanting to make fifteen types of profit from one product, they are not hygiene essentials.
I reckon I can go away for a long weekend with just a handbag. My luxuries are disposable contact lenses and a netbook because internet.