Can you give a little more info on what type of tour you want. For instance, do you want mountains, desert (probably not in the summer!), open plains, rain forest, historical, etc. Can you give a location in Europe so we might get an idea, i.e Holland or southern Italy.
What about temperature? Since you are from Holland, you probably are not used to our summers where in many parts of the country it can top 33 degrees celsius every day while other areas never get above the upper 20s.
When you say "July/August", do you mean 60 days or 7 days during that time period?
Do you want dedicated cycle paths (most will be finely crushed gravel coverted railroad paths and the longest is less than 500km long), rural country roads, big cities, etc. When you say "relatively flat", are you talking road grade or geography as the long shallow grades Colorado roads are easier (to me at least) than the lung-bursting thigh-burning short-but-steep roller coaster hills of Missouri.
If you are as strong as a Holland grandmother rider we met (BIG SMILE) you will have not a problem with ability. I was doing a tandem tour in Holland and one day a 70+ year old lady passed us on a single-speed upright bike into a strong headwind. That humbled and impressed me.
The answers will help everyone not second quess what you are looking for and hopefully suggest a wonderful ride.
Happy trails and may the wind be at your back!
TulsaJohn