joelle,
The Guidebooks have lots of suggestions on sightseeing possibilities in each of those locations. I'd suggest packing along a copy of the Italy 2010 book, as I find it's good reference during a trip.
I wouldn't bother renting a car. It will not only be slower than travel by train, but you'll also have to worry about parking, going through ZTL zones and you'll need an I.D.P. for driving in Italy. All of the areas you're visiting are easily covered by train.
A few suggestions on some of the locations you'll be visiting.....
Milan - visit the Duomo (including the roof), Galleria Vittorio Emmanuele, browse the Rinascente stores, Sforza Castle and the Last Supper (if that interests you - book early).
Stresa - there's MORE THAN ENOUGH to keep busy here for 3 nights! Tour all of the Borromeo Islands (or just Isola Bella, which is the largest - Isola Pescatore is mostly just restaurants, although there's an interesting Church), take the Cable Car to the top of Mt. Mottarone (great views) and for a wonderful day trip take the incredibly scenic Cento Valli railway to Locarno (Switzerland) and then return by boat (be sure to take your Passport, "just in case"). It's a long day, but a really nice trip.
Happy travels!