To me the essentials are a sweater and a jean, because wherever you go, it can be cold at night, clothes corresponding to the weather and that you are sure to wear, swimsuit and sunscreen if needed, and a towel.
For the beauty products, just something to clean and moisturize your skin, hairbrush, toothbrush and toothpaste and that's it.
The trick is to avoid packing useless stuff, like for exemple don't take any make-up, nor ten skin creams o hand creams, you won't need them for three days.
A useful trick can be to pack some flip-flops, they are light and don't take too much space and can really be useful in a lot of situations.
That's the biggest struggle when I go on a city trip, when I usually just have a bag pack.
I look at the weather and take just clothes I am sure to wear, nothing more.
For beauty products: my basic skin care routine in miniature format, a mascara, a lip balm and that's it! Add a sunscreen if you go to a sunny destination.
Don't forget your passport and your train/plane tickets of course aha