I just spent a lovely vacation in the Ardennes with my family and had the chance to cycle almost every two days. The weather was great, mostly around 26 degrees. I have mapped out some of the better climbs and routes I took.
I rode a total of 135 km spread over 5 rides, 52 km was the longest, and it was either climbing upwards or descending downwards. I was unable to find any flat roads at all. I spent the better part of my best rides in the 180+ heart rate range, just loving the steep hills.
See the map for details about the better climbs I found.