21 Day Tour

In mid-July it's going to be very warm to hot everywhere except for in the mountains where it will likely be pleasant so pack accordingly, maybe throw in a light sweater for evenings. You will want to pack some kind of rain protection (light jacket, poncho, etc) because it can rain anytime although less chance in July then other months. Be sure to pack a hat of some kind since the sun will be be brutal at higher altitudes even though temps may be lower. A pair of good walking shoes and a pair of sandals. Mid summer is the easiest time to pack for, lighter weight clothes and fewer layers.

Should be warm, but one can never be sure. Rain protection is a must and so is a good sun hat and good walking shoes. The key (imo) is layering with a variety of long & short sleeve tops, lighter weight and heaver weight pants, light sweater, and light jacket. If it turns really cold (though unlikely) you can always buy something there.

I am not sure whether you are male or female but if you are a female I recommend 2 pr long pants (wear one of the plane) and 2 pr crop/capri pants. If you wear skirts, sub in a skirt for one of the capris. Italy is going to be warm but on some days you will also need your knees and shoulders covered for when you visit churches. If you are going to wear sleeveless or tank tops make sure you have a large scarf in your purse that you can throw over your shoulders to enter a church. I would also be ready to flex your wardrobe right before you leave depending on if Europe is having a cool vs hot vs medium summer.

I second the need for rain gear. When I did this trip I took a Marmot Precip which I could fold up and put in a ziplock bag in my purse so it was always at hand. If you are going to purchase something get waterPROOF not water resistant. I like mine to have a hood and to be long enough to cover my behind.

As hot as it may be, if you are planning to wear athletic shoes with socks, I recommend you invest in merino wool. They actually are not hot, then don't get smelly and if you get the ultralight weight ones from Smartwool, will dry overnight. Many will recommend sandals but my feet won't tolerate that, I have to wear athletic shoes. I would go for ventilated ones.

Definitely a sunhat for Italy. There are tons of vendors in the St Marks Square area where you can pick up a cheap one for under 10€ if you don't have one to bring with. It'll last you the rest of your trip and perhaps make it home, lol.

I loved, loved, loved this tour! I am still FB friends with about a dozen from my Fall 2014 tour.

Do you have your airfare yet? Do give yourself at least one extra night in Haarlem before your tour starts, 2 if you can mange it time and money-wise. If you can manage another night or 2 in Paris afterward go for that too!

edited to add: A link to my Trip Report. It was 2014 and a bit later than your trip but the other random advice should be still good.

https://community.ricksteves.com/travel-forum/tours/21-day-best-of-europe-8-26-14