Hiking in the Poschiavo Valley is very popular and there are lots of hiking paths. Just follow the yellow arrows. The signs usually indicate the approximate time to the destination. If there is a white/red/white tip of the yellow sign, then it is mountain path which needs more serious preparation like proper hiking boots, a map, snack, water bottle and preparation for weather changes and possibly a reservation for beds in a mountain hut.
One of my favourite hikes of all time! This valley is not overrun with hikers at all due to its relatively remote location beyond the Bernina pass, seen from the Upper Engadine. Getting there is either by car or bus. Park the car at Sfazu on the Bernina pass road (1640 m).
I have hiked from there into the valley a couple of times - it's a quite long ascent (about 1:30 h) that you can save by taking the small bus (reservation in advance) to the mountain hut Rifugio Saoseo (Lungacqua) at 1985 m. From there the valley floor is relatively flat, the hike to Lake Saoseo is easy - not more than a half hour of strolling through open woods (Swiss Pine mostly) and often blooming wildflowers. The lake is breathtakingly beautiful: unbelievable blue colours that seem to be not from this world, blooming Alpine roses around it, mighty peaks towering up above it. Walk around the lake to the opposite end - and you'll see the glacier of Peak Palü in the background (pic 5).
If you have enough time continue your hike to Lake da Val Viola at slightly higher elevation. It's only a bit more than an hour round trip. This lake is different, already on the timber line so the woods are smaller and rarer, the colour is also different.
For the way back take the smaller hiking path on the southern side of the valley that takes you through beautiful forests where blueberries grow (had some once :-)).
It's been a while ago that I've done this hike. Thus only crappy pictures, sorry :(
Take the small bus from Brusio's train station up to the small village Viano. From there turn left and hike along the mountain slopes, always slightly ascending, to Alpe Romerio. There is a Romanesque chapel (nothing special) at this Alpine pasture, cows grazing, milk and cheese available. The views from there are stunning - you overlook the whole upper valley to the Bernina massif. The descent down to the lake is quite steep. From the lake you have two options: either the short walk to Le Prese (with hotels and cafe by the lake) or the longer hike to Poschiavo (almost an hour longer). All in all the hike is about 5 hours, not counting the breaks. So start early :-)
This is another hike that I've done some years ago, thus no pictures taken right then. Instead you see pictures of the hiking path and the area I took from the train in 2011.
Ospizio Bernina (2256 m) is a good place to start hikes. To get to Alp Grüm (2091 m) you have several options. The nicest IMO is the trail via Sassal Masone that first leads along the lake (Lago Bianco), then turns right in a medium ascent. Sassal Masone is an Alp with a restaurant where you find surprisingly great (and expensive) wines - even Grand Crus from Bordeaux! Odd. Anyway, worth a look are the round stone huts that look like the Trulli in Southern Italy. Here they work as fridges for the milk from the cows.
Continue the hike along the mountain slope toward the Palü glacier. Take the first trail in a hairpin curve back, descending slightly to Alp Grüm where you can catch regional trains back (hourly). All in all the hike is about 2 hours without breaks, easy.