I’m an Italian from Milan
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What’s too long a car drive?
- There’s plenty of castles in northern Italy but Milan already has a beautiful castle, Castello Sforzesco. It’s in a park so you can walk around and through it for free. Inside there’s some paid exhibitions but they might be boring for a child, depending on taste.
- A museum that children usually love in Milan is the Leonardo da Vinci Technology Museum. Among many things, it’s got models of Da Vinci’s projects but also lots of old trains, planes, ships, a submarine.
- A fun thing in Milan is also the elevator or stairs to the Duomo rooftop.
- For seeing the Last Supper make sure to book months in advance
Any tourist destination in Italy - especially seaside and lakes - at the end of July will be packed with people - just be aware of it. The more famous, the more overtourism it gets.
Garda
You mentioned lake Garda and Gardaland, every major town there has a castle. A particularly spectacular and well preserved one is the one in Sirmione which is right on the lake.
The area near Peschiera del Garda has several amusement parks. Near Gardaland there’s also Caneva World (which has a waterpark and a few other parks, including a medieval one with a medieval dinner).
Every town is pretty on Garda. Peschiera, Sirmione, Salò, Lazise, Garda, Bardolino.
Riva del Garda, Torbole, Malcesine, Limone are very pretty but a farther drive out.
From Malcesine there’s a cable car to Monte Baldo for a taste of mountains.
Lake Maggiore.
The nicest towns are Stresa, Baveno, Cannobio. Arona also deserves a stop. Boat trips to isolee Borromeo are worth it. For a castle Rocca di Angera. Mottarone (the local mountain) has nice views, easy hikes, you can drive to the top (it’s also a great cycling climb). Nearby Lago d’Orta is beautiful too (Orta San Giulio and Pella, boat trip to isola di San Giulio).
The swiss side of lake Maggiore is also really nice (Locarno and Ascona).
Lake Como
Lake Como isn’t overrated but it’s not more beautiful than the other lakes and traffic to/from can get terrible. Because of its shape, you also end up spending a lot of time driving or on a boat.
If in Como you can take the funicolare di Brunate (go early or the line gets crazy) and there’s a couple of hikes at the top.
If you end up driving to Bellagio, do stop at Madonna del Ghisallo. Beautiful view, famous climb of Giro di Lombardia. The church (and also the museum in front of it) is full of bicycles and cycling memorabilia.
Iseo
Another lake, lago di Iseo is also very nice. Less famous, more compact, just as pretty. Wine region.
Liguria
Cinque terre is a truly outstanding and worthwhile destination but over tourism might make it uncomfortable.
For a taste of Liguria with a shorter drive you could consider Camogli, Santa Margherita and Portofino. Try to avoid the typical weekend flow (Friday afternoon going to the sea, Sunday afternoon going to Milan) although with your schedule it might not be an issue.
Parma is a pretty city so worth a stop if going for Parmesan cheese making visits near it.
Bormio Is a beautiful mountain destination that is a bit closer than dolomites. Still 2.5h driving from Milan. Nice hikes and cycling, can easily rent, too. Could also add trips to Livigno and/or swiss mountains from there (st Moritz).
Other ideas:
- Monza (15 min from Milan). Duomo, old center, royal park and palace. Formula 1 track.
- Bergamo alta (40min). Beautiful hilltop old town.
- Pavia (40min). Castle, old town.
- Verona (1h 45)
- Mantova (2h). Easy to add if going to Verona and Garda.
- Lucca and Pisa (3h). Pisa a bit overcrowded. Lucca is a gem.
I would avoid Venice in summer. I think it’s 1000% worth it, just not in summer.
Some destinations are easily reached by train also.