The Orrido di Bellano is a natural gorge located in the town of Bellano. The gorge was created by the erosion of the River Pioverna and the Adda glacier. It's possible to visit the narrow canyon via footbridges anchored on high walls, offering stunning views of waterfalls and rock formations.
To see the Orrido di Bellano it will take around 15/20 minutes and the adult ticket is €5. No need to purchase the tickets in the advance, you can buy them directly at the entrance.
Personally, I really liked it, but it's not a top priority location on Lake Como for me. It's nice to see if you're in the area. It doesn't take a lot of time to visit, so you can see it by doing this if you're coming from Milan:
Take the train from Milan Centrale to Bellano, visit the Orrido di Bellano and then take the train to Varenna to visit the village and the villas
If you're in for a short hike, the trail of "Sentiero del Viandante" toward Varenna starts nearby. See the nearby pin I made about it :)