There’s a fundamental error here. It’s an understandable perception, but it’s just wrong to assert that you have to visit the major museums to see great art.
Yes, it’s true that those well-known museums — the Louvre, the Uffizi in Florence, the Vatican galleries in Rome — are very crowded. They’re stuffed with amazing things, but you do have to shoulder your way through lots of people.
But you don’t have to go to those museums in those cities. We’re absolutely swimming in great art here in Europe, and there are many, many other options besides the ones Americans think of as the Top Ten due to their heavy promotion. When we were in Lucerne, Switzerland, for example, we visited the Rosengart to see their Picassos. That’s what they’re famous for, but it turned out their collection is extensive beyond that — Klee, Mondrian, Bracque, Cezanne, and others, including a couple of Kandinskys I hadn’t previously seen (probably my favorite artist). And best of all, we had the place almost to ourselves. There were probably ten or fifteen people in the whole museum while we were there. And it’s hardly unique.
So please don’t tell people they’re out of luck for visiting excellent museums and seeing excellent art unless they’re willing to brave hordes of other tourists at the famous galleries in big cities. It simply isn’t true.
My suggestion to the OP, if you’re considering Europe — take a serious look at Slovenia.
It’s barely known in the US, and when we were there, we met only one other American couple. The tourists are mostly Italian, with some Germans and French and Hungarians, plus a few from the UK. It’s really under-visited by tourist destination standards.
But it’s gorgeous. The landscapes are astonishing; the country sits at the fringe of the larger Alpine range, and their mountains and rivers and valleys are largely unspoiled. My wife and I spent lots of time hiking different parts of the Triglav National Park during our holiday, including the trail that visits this heart-stoppingly beautiful waterfall.
Ljubljana (good luck pronouncing that without practice), the capital city, is small by metropolitan standards; you can thoroughly explore the whole thing on foot in two days, or three or four if you include a couple of the museums and other cultural sights. And it’s not crowded at all.
There’s also the fairy-tale postcard of Lake Bled, the wine country of Maribor, the beautiful beaches of the short southwest coast… it really does have everything. You can spend days alone in nature, and then easily dip back into city life for a bit, and then get back out of town. It’s not on the typical American’s travel radar, but I can’t recommend it highly enough.