I'm going to Paris, Amsterdam and Nice. What kind of clothes are appropriate?

Whatever you would normally wear in the summer at home.

A reasonable answer, unless OP lives in Florida or similar.

For Amsterdam assume you will need a lightweight waterproof (or an umbrella), and possibly a lightweight cardigan.
Hopefully you won't need them, but better safe than wet or cold.
You can always check the weather forecast nearer the time.

Wear what you are comfortable in, don't worry about trying to blend in - you won't, and nobody in those cities cares anyway - they are all too used to tourists to notice one more.

Since the summer sales start in July, bring fewer clothes and go shopping while in Paris!

Presumably you'll be doing a lot of walking so be sure to bring comfortable, practical, well-broken-in shoes. Save the cute, dressy shoes for an evening out. Your feet will bless you.

A tissue weight rainjacket with hood and a sturdy folding umbrella. A light cotton sweater. Two pairs of waterproof comfy walking shoes. Summer clothes you would wear at home (but be aware that it might be too chilly for shorts - and I don't like them in cities) - although you can also get highs in the 90s, esp in Paris and Nice.

I would recommend that you begin checking the weather a few weeks in advance and then plan accordingly. I spent a summer in Paris with weekend trips to Amsterdam and Nice in late June. Amsterdam was very cold compared to midwestern American summers, and I actually wore a sweater with a light jacket most of the time. However, I have no idea if that was typical or just an abnormally cold weekend.

Paris in the summer can be all over the place, it might be in the 90s and miserably hot or rainy and cool. You really just have to check the weather. Also, keep in mind that it's fairly common for places to not have air conditioning, so 80 degrees in Paris feels hotter than 80 degrees in the U.S.

Nice has a mediterranean climate, so expect it to be significantly warmer than Paris and much, much warmer than Amsterdam.

Here is what (non tourist)people were wearing in Paris a few years ago. Draw your own conclusions about what is acceptable.

http://anyportinastorm.proboards.com...ay&thread=3657

The weather in Paris is fairly similar to that of New York City or Washington DC.