Even in the height of summer I was just about the only person at the Prieuré & I passed a lovely afternoon in the sunshine, wandering around the ruined priory, the beautiful gardens and the exhibits inside. While I was there, there was a gorgeous exhibition of decorative art by Zao Wou-ki, who did the new stained glass windows in the refectory. A lovely day trip from Tours! Btw, I walked there and back from the Halles area, about 30-45 minutes each way depending on how fast you walk (and easy to find, even for a dunce like me without a map), but it is easily accessible by bus as well.
Photos and more on my trip http://gwannelsandiego.blogspot.com/2010/08/prieure-de-saint-cosme.html