There is a ferry in the town of Luino that takes you across the lake to Cannobio. It is a very charming little town that's walkable and as it's very near the Swiss border, it has a different feel than other Italian towns. Be sure to check the ferry schedule, as there aren't trips across all day long.
On Wednesdays, there is a fun market in the center of Luino. This could be something to do the morning of the wedding.
We went here and highly recommend going if you will be in Milan at all. You need to reserve tickets months in advance, so please check on reservations beforehand. Tickets for August and September become available on June 18th. You only get 15 minutes inside to see the Last Supper, so make sure you are there at least 30 minutes beforehand.