Favorite thing: I strongly recommend that you travel by train coming from the north if possible. Making the steep descent the views are spectacular. You will see the valley and Poschiavo from almost a bird's eye view as well as the lake and the distant mountains which lie in Italy. It gives you a feeling why there is this unique culture here in Switzerland.
Favorite thing: In summer and autumn the Poschiavo valley is very popular for hiking. What I like, is looking at the mountains and the sun playing with them when it rises or sets.