Hello Eileen,
First question - this is next year not in 30 days correct? If not the you need to settle on a schedule and tackle lodging ASAP. September and early October is still well in season for lodging, tickets and transportation.
I would give Milan a couple of days since it has a number of interesting places but it is a large busy, modern city for the most part so your mileage may vary for how long you want to spend there.
Lake Como is easy from Milan by train. Varenna is highly recommended for good reason but be aware that it comes with steep hillside ascents and descents often on uneven and handmade paths.
https://c8.alamy.com/comp/S129GC/steep-cobbled-steps-in-varenna-italy-S129GC.jpg
(This is not an exaggerated photo - I stayed in the room that has that top arch balcony.)
Como is the big city at the bottom of the lake so you might head there and if you want something quainter take the ferry up lake. Bellagio is the ritzier upper lake town but it is not without its hills and stairs. You can reach it from Como or faster from Varenna walking downhill 10 minutes from the train station to the ferry port. Bellagio is also well connected by ferry to nearly everywhere on the lake.
You might also look into Lake Garda which is closer to Bologna, Verona and Venice. Access to the upper lake is a little more difficult without a direct train but the lakeside towns are generally somewhat flatter. All three of the big lakes are beautiful in slightly different ways.
If you like architecture and history larger city recommendations are Verona and Padua and the smaller Vicenza. You could stay in any of these - I would suggest Verona - and day trip to the others. If you're in Bologna there are the obvious food towns of Parma, Modena and Reggio Emilia. And while I haven't been Ferrara is on my list that can be a day trip from Bologna. Florence is also only 30 minutes by fast train from Bologna.
Venice as a town is flat but there are the occasional bridges - all slightly different in terms of steps - but more importantly all transportation requires the stepping into and out of boats. This is generally easy but sometimes rougher waters or odd dock heights can make this suddenly a challenge. The people who run these things are aware but especially handling bags under these conditions can be a challenge.
That's not to discourage you from going because I love Venice and think it is amazing, but it is logistically a hassle for long term stays. I would recommend at least 3 days in Venice with the time you have. Add more time and explore some of the islands in the lagoon.
Venice to Milan is easy by fast train but arrive the day before your flight minimum. MXP - the most common Milan airport to use is an hour outside of Milan itself by train.
Have a great trip!
=Tod