Idea was to restore too bright, but not completely blown highlight in the sky and not darken the whole picture. To do this I first selected sky (using any method you prefere). In this picture it was not particularly difficult, because of distant edge between sky and trees on horizon. After that I copied selection as a new layer. Next step was to change blending mode from "normal" to "multiply". This is usual technique if you want really darken something. After that I've got better contrast and saturation on the sky. Next step was overall saturation boost and a bit of hue shift +3. It usually "pops" a bit colors. I also used "shadows and higlights" tool to lighten shadows a bit. After that I ended up with pretty noisy image, since all boosting decreases signal/noise ratio. I applied also some USM sharpenning and then run NeatImage to clear all noise. The sky ended up to be a bit soft, but still not bad.
My suggestion for this kind of shots will be using split density ND filter and put split like at the horizon level.