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A friend and I did a morning's mountain flying from Annecy last year (with an instructor) and it was great fun. We did Megeve and Courchevel and one altisurface. The latter was a very steep rough meadow with a flat bit at the top.
Whilst I would thoroughly reccommend it, I would not suggest you try without an instructor (whether legal or not). Just one example of why: taking off from the altisurface, we did not really have flying speed at the end of the strip, where the mountainside fell steeply away below. The trick is to push forward and dive into the valley to gain flying speed, whereas conventional wisdom is to pull back to take off. The latter would have caused a stall.
Density altitude performance, unusual circuits, escape plans are other reasons which I learned after only a couple of hours.
Enjoy, I'd like to go back.