If you're only doing airport-taxi-hotel with your backpack, then no problem, bring as much stuff as you want.
Otherwise, you'll have to think of the weight. You have to aim for a 25-30 lbs backpack, 35 at the most. Really, believe me. The lighter, the best.
So usually, I have clothes for 3 or 4 days, that's all. You handwash twice a week or so.
You want to look good ? 1 pair of sandals you can go out dancing or walk around with, 1 pair of jeans, 2 light cotton skirts, 2 nice long sleeve sweater, a combination of 3 of these : beautiful t-shirts and/or no sleeve shirts... plus the usual windstoper-raincoat jacket, 1 pair of pants-shorts with zippers (don't know the name in english), and a good pair of light boots or sport sandals for loooooooong walks of discovery (but in springtime, I would choose the boots, I think). The key is "light and fast to dry".
In most countries of Europe, shorts are not wore
but bermudas yes, and discreet colors will help you not look too "touristy".
You'll be surprised how fast your backpack will gain weight... The first time I packed, it was 45 pounds, and I thought I had nothing. So I took out the surperfluity, and kept.... for 3 or 4 days of clothing.