The Alpes would be a great destination. There are a couple of hotspots which enable you climb a lot of "big names" from the same base.
- Bourg d'Oisans (France) : Alpe d'Huez, Col de Croix de Fer/Glandon, Col de Galibier, Lauraret, Télegraphe, Col du Sabot, Col d'Ornon, etc.
- Barcelonnette. Col d'Izoard, Col de Vars, Col de Cayol, Col de Bonette.
- Andermatt: Furkapass, Gotthard, Grimsel, Nufenen, Lukamanier, Sustenpas
- La Villa/Corvara: Sella Round (Passo Sella, Gardena, Pordoi, Campalongo), Fedaia, Giau, Passo delle Erbe, Falzarego, Valporola
- Bormio (Italy): Stelvio, Mortirolo, Gavia
The Pyrenees would be another good option but the weather is a little less stable.
Switserland is great to bike but please note this is very expensive. A simple meat would be somewhere around the 30€ mark.
I wouldn't recommend my own country (the Netherlands): no mountains and the weather can be very variable.
I wouldn't recommend Belgium since the roads are horrible (3rd world quality sometimes). In the Ardennes the weather is also very variable.
Bedoin, at the foot of the Ventoux could also be a good option. You'll most certainly have good weather but it's going to be very hot in August.
I rode Ventoux last June. Very tough climb. Great challenge, not the most scenic Provence is nice and you can take an excursion to visit some famous Rhone wineries or do an easy ride to them. Problem is those mistral winds so its quite possible you arrive to ride and can't really ride it. I'm not that familiar with the other routes in the area. Ventoux pretty much dominates. Many come for just a few days and climb it from different directions.
I'll be doing the French Alps in June. It was on my list of bucket rides.