Beyond Lake Como's Big Three

No specific town to recommend along this route, but if it's a clear day with mountain views, then taking the ferry north to Colico will reveal a few more mountain peaks (not quite the same as being in Switzerland, however).

We stayed in Varenna and bought a 1 day boat/ferry pass at the ferry landing. We went to every town that the ferry stopped in and enjoyed a walk around each one of them. In Bellagio we bought cheese and bread and bottle wine and sat on the grass in the park and had a wonderful picnic.
I think you will have a nice amount of time to get on the ferry and at least stop in the ones mentioned in your post.

If you enjoy visiting lakeside villas surrounded by gardens, don't miss Villa del Balbianello located near Tremezzo. This villa is very unique compared to others in the area. For one thing, on the top floor you will see memorabilia from a Mount Everest expedition which the former owner participated in. When you get off the ferry in Tremezzo, walk about 5 minutes to the left and you will find a small dock for a private transfer in a little motorboat to the villa. Check before you go to make sure the villa isn't closed for a private event since it's a popular wedding venue. Como isn't as picturesque as the smaller more northern towns that you mentioned (Varenna and Bellagio). If you can, try to spend a night or 2 in Varenna (perhaps instead of Como) to give yourself more time to visit this beautiful part of the lake. While staying in Varenna, I enjoyed taking a walk along the water in the evenings...very peaceful. Varenna also has a fairly good variety of restaurants. I splurged one night on dinner at Villa Cipressi which was well worth it. Ristorante il Cavatappi is another great place for dinner in Varenna.

We ferried over to Menaggio and enjoyed this town. Como was okay. The scenery and photo ops in Varenna and Bellagio were the best, however ! It is fun just to take the ferry ALL around the lake and enjoy the views, stopping at any location that catches your eye.

In Varenna, we enjoyed the Nilus Bar, eat sandwich #24, and Il Molo, right next to Nilus, and eat #5 and #7. This is NOT fine dining, but delicious sandwiches while viewing the scenery of the lake - spectacular !

Thanks for the suggestions, everyone. My day on the lake is taking shape. I hope the skies will be clear. I'll be there in mid-October, so fingers crossed!