Lake Como

Hello! How many days will you be staying in Varenna? Is this your first trip to Lake Como? Not sure if you have considered visiting nearby towns on the Lake such as Bellagio. Also a worthwhile day trip to visit nearby Villa del Balbianello is worth considering. Both of these destinations are reachable by public ferry, although the Villa requires a walk from the ferry dock which can be rather long, or there is usually a small private boat transfer available.

Personally, I would reconsider Bologna as a day trip from Varenna as it would be at least 2.5 hours one way by train and approximately 3 hours one way by car. On my first trip to Varenna years ago, I spent 3 days just exploring Varenna, Bellagio & Villa del Balbianello. If you enjoy visiting villas with beautiful gardens, you will find lots to do in Varenna

We are stsying for 8 days. We plan on exploring the towns around the lake

I would also reconsider Bologna due to distance. Consider Bergamo instead, the old town up on the hill is interesting.
More subjectively, I am not a huge fan of Lugano either (I prefer Lake Como), so I would prioritize the Bernina railway.

I have heard that Monza is also worth a visit. A friend of mine has stayed there & loved it. It's famous for its Formula One racetrack. It's less than an hour one way by train (direct/no transfers) from Varenna.