northern Italy

How much time will you have in Italy? Are you wedded to the idea of spending the whole time in one place? Unless your trip is very short, you might do better to stay in two or more different towns so you can explore more than one area.

I think day-trips work best when the destinations are fairly small so you don't end up feeling you need to make multiple trips to the same place. I'm not sure I'd want to day-trip to Lake Como, just as an example. It seems more the sort of place to spend 2 or 3 days so you can get to several of the lakeside villages.

I think Verona-Vicenza-Padua make a nice cluster of spots visitable from each other, with the bonus that you can use the train rather than driving.

The classic area, of course, is Tuscany for the hill towns.

We are doing the northern part of Italy in May 2018 as well. We have been to Verona before & would highly recommend it for at least a 2 day visit. We will be doing Padua for 2 days this trip as well and Venice for 2 days(been there 4 times before so this will be just to relax). We will be heading due north to Bolzano & Caseltrotto for 3 days and driving over to Cortina. You may want to include these as well. I do not know how long you are spending in Italy.
Verona is close to Lake Garda which is a beautiful area and IMHO far outdoes Lake Como.

I thought about mentioning the Dolomites but wasn't sure what sort of weather would be expected in May. It was great when I was there in July 2015.

Thank you for your replies. They are a great help!

I'm only a few years behind you. Since you are renting a car, are you open to driving south toward Sienna? How long is your trip?

We are thinking about three weeks.

You won't be able to day trip to Pisa if you base up north, it is too far. Is your intention to stay in one place for the full three weeks? If you are worried about too much difficult walking, hillside villages may not be your thing. I liked the lake district (but nowhere near as much as Tuscany and parts of Umbria).

My wife and I spent 3 weeks around the lakes a few years ago, half the time based on Lake Orta and half the time based on Lake Como. We did plenty of hiking on the side but the towns and villages are all relatively flat. Orta San Giulio and it's tiny island are fantastic, day trips from there to the west side of Lake Maggiore (Locarno, Stresa, the Boromean Islands, the garden of Villa Taronto), some twisty driving to catch the Centovalli Railway, Turin to the south, the little gem Bergamo about 90 minutes away to the east, Certosa di Pavia and the Abbey of Vezzolano may be of interest. Note, the driving immediately south of the lake region is boring and heavily industrialised.

Based on the west side of Lake Como, we spent some time hiking again but there is time for relaxing on the boats if the weather was agreeable, the city of Como itself is worth visiting (though I would not return), Bellagio and Varenna are very pretty, Lenno has a great market day at the lake side (Thursday?), and then there are the famous villas: http://www.villadeste.com/en/124/villas-of-lake-como.aspx

Lugano is a pleasant half day. A lot of people like Menaggio, I did not.

Drive to the Bernina Express for a long day out, if the weather holds out - it did not for my wife and me, we had to give it a miss. Make sure it's not cloudy if you decide to give it a try.

Bellinzona (Switzerland) is a fantastic day trip, a nice old town and 3 castles. The castles are a bit of a hike but I believe you can drive (bus from town centre?) to all of them.

My favorite lake is Maggiorie. Nice small towns to visit/stay in around the lake are Stresa, Baveno and Cannobio. The drive around the lake shore is beautiful and there are islands in the lake that you can take boats to for lunch or a visit. You can do the Bernina Express (combo train/boat ride) in one day and their is a cable car in Stresa that take you up the mountain and there are gardens to visit. If you want a larger town up North, Verona is wonderful. Email if you need hotel recommendations or any other information.