Top 5 things to & 3 things not to do.
I was just in Como last week.
I would highly recommend taking the slow boat to a town up the lake - I visited Bellagio which was wonderful. It takes about 1.5 hours and you can stop and have a beautiful lunch at a lake side restaurant and then walk up the steep hills and discover parks/hideaways.
Yes definately get the ferry around the lake. It is just beautiful. Spend the day in Bellagio - it's so pretty and there are some great restaurants. Visit Villa Charlotta. It is a magnificent old villa with stunning gardens. It is full of azelas that when in flower are amazing. Still worth a walk around the gardens even if they aren't in flower. The villa is worth seeing too. Have a look at the tripadvisor web site under Lake Como attractions. They list several that may interest you.