Here's a particularly mind-blowing route that starts in Val Grosina, just above Grosio. From there, a number of tracks and trails head up the valley towards Rifugio Eita. (There's also a road with restricted access but the trails on the righthand side of the road are much more interesting). From Eita, continue in the same direction and run and over Passo Verva along a gravel track. Drop down the other side and you're into Val Viola. Swing left, follow the single track and keep going until Rifugio Federica in Dosde.
Tip: sleep here and carry on the next day.
Once you've regained your energy at the rifugio, head up towards Passo Dosde at 2,800 metres. It's pretty amazing. A slightly techy descent to a lake (the deepest in the Alps), before the downhill gets super runnable towards Malghera. This is where you might start meeting other hikers, but you'll be going to fast back to your parked car at Fusine that you won't notice. A great way to round off a hut-to-hut.