“A dedicated (paved) bike path runs up the beautiful Valtellina Valley from Lago di Como to Tirano.”
— Lost Justpastnowhere
The Sentiero Valtellina is a paved (dedicated) bicycle route that runs from Tirano to the mouth of the Fiume Adda (river) to Colico on the Lago di Como (lake). The beautiful Valtellina is hemmed in on both sides by steep 2,500 m (8,200 ft) and higher mountains. The sides of the south facing (north of the trail) mountains are lined with acres and acres of vineyards and the occasional old castle. The trail passes through a number of villages along the way offering opportunities to enjoy the local cafes and bars.
While certainly not a mountain biking trail, the Sentiero Valtellina can be used to connect a number of mountain biking routes that climb up into the mountains, although those seeking singletrack bliss are likely to be disappointed.
A rail line also runs the distance of the valley (final stop in Tirano), making it possible to use the train as an out-and-back route. Although be sure to check the train schedule if you're planning this and keep in mind the bike will require a separate ticket.
Along the route there are a few places for bicycle rental (presumably road bikes though) and lots of picnic areas.
In Colico, you can connect with the boats on the Lago di Como which stop at many of the cities along the lake shore (as well as a number of other MTB trails).
In Tirano, an extension of the Sentiero Valtellina runs an additional 40 km (25 mi) to Bormio, although about half of this distance is actually on paved roads open to vehicular traffic. It's also possible to take the Bernina Express train from Tirano into Switzerland where it connects with its namesake trail (the train was there first) and eventually to the excellent riding in Graubunden around Davos and St. Moritz.
for details, although unfortunately you'll have to speak Italian.