Judy,
One of my favourite "activities" while in Varenna is just relaxing and enjoying the town. You might enjoy a tour of the gardens at Villa Cipressi & Villa Monastero (as I recall it's possible to get a combo ticket that covers both).
The Ferries are "easy to use" and the dock is located in front of Albergo Olivedo (bright yellow, you can't miss it). The trip to Bellagio is about a 20-minute ride.
You might try dinner at Albergo Olivedo one evening. If the weather is nice, it's great to sit on the patio watching all the traffic come and go from the Ferries. Laura's Ravioli is excellent!
Happy travels!