Palazzo Borromeo is a 17th century residence in the middle of Lake Maggiore. Inside the residence there is the Teatro Massimo which is the most important monument in the garden of Isola Bella.
Lake Como is about a hour car ride from Stresa. There are also trains going there. We recommend visiting Bellagio during your time in Como.
Visit the Duomo and stroll around Galleria Vittorio Emanuele for some shopping. If you walk about a mile on Via Dante you will pass the old Italian Post Office building - which is now home of the Starbucks Reserve Roastery - and then you will find yourself in front of the Castello Sforzesco. Yes we have a castle in Milano! you can hangout at the park or go visit the castle, they have galleries and museum visits. Go down to the Navigli/Darsena area for a nice riverwalk aperitivo (order an Aperol Spritz of course!).
If you are a Leonardo Da Vinci connoisseur then you don't want to miss out on seeing the Last Supper. It is conserved inside the Church of Santa Maria Delle Grazie.
Luini makes some of the oldest "panzerotti" in Milan. It has been around since 1888. They have numerous fillings so you can try and find the one you like best