The Sentiero Valtellina is becoming increasingly popular, and it is a cycle path that does a great service.
However, it is designed for the summer season, so it is close to the river, cool and shaded, and this makes it less practicable in the bad season.
There are several alternatives and variants, but the Chiurasca/Valeriana road and the cycle path area between Chiuro and San Giacomo are one of my favorites. With little up and down, they stay on the valley floor and connect the villages between San Giacomo and Sondrio, crossing apple orchards, countryside and lesser-known parts of Valtellina, offering panoramic views.
Very quiet, not very busy, I like to ride it even just for variety.