I've just cleared the Valtheim Towers from a river bellow with my DB character and it was so much fun. Do you know any places like that, where you can sneak up and ice them [censored]es from a distance without being seen?

That's like 20-25% of the places in Skyrim. Most of the Foresworn holds/camps (most are in Haalmarch or the Reach) are like that. Often if there is a rock cliff you can scale it from one side or the other a bit and get above them too.

Only if you're lucky enough to not have your shot blocked off by invisible walls and third-person killcams. Other than that, sniping is so much fun in Skyrim.