It's been 15-20 years since I last visited Europe, so these recommendations may be a little dated. That being said, I recommend the following:

- Try to avoid clothing with any names on it, unless they're from a European manufacturer. But any generic clothing is probably OK. One glaring exception was Levi jeans. Europeans love Levis and Zippo lighters. But generally speaking, wearing khaki slacks should be fine. As for shirts, I think most Polo shirts should be OK for most daily use. Once again, probably wouldn't wear anything with a major US brand, but even Ralph Lauren is probably pretty common over there. The same goes for a rain coat. A Rab (UK brand) jacket would probably be optimal, but I think most "hiking" jackets probably blend in now.

- When selecting walking shoes, I recommend wearing a European brand, such as Ecco, Mephisto or Dansko.

The best compliment I received was when I was in Marseilles and a French couple walked up to me to ask something in French. I knew I succeeded in my quest to "blend in."

None of this is mandatory, and most is probably overkill, but you never know...

Also another good thing to do is to learn a few phrases in the primary languages of the countries you plan to visit. It shows that you are making an effort to communicate, and don't expect everyone to just speak English. That would label you a "ugly American." in their minds...lol

Also don't be surprised by how much Europeans smoke. They seemed to smoke 10X as much as Americans. Clubs were especially horrible.

If you see something that looks like a shortcut through a "seedy" part of town, avoid it! Stick to the major tourists areas. They were bad back then, and I'm sure they are much worse now after the huge migrant influx.