American tourists in Europe often make a point to visit historic cathedrals, and for good reason - they have a grandeur that most of ours lack. (If anything, the American equivalent might be our state capitol buildings, even if they are not religious buildings.)
In most cases, I’d say that it isn’t particularly important exactly which churches you visit, as most cities have several grand old churches with extravagant interiors that you can walk into. But there are a few that stand out, and one of them is the Basilica of St. Francis in Kraków. While it’s an old church, it was damaged in a fire in the 19th century.
That created an opportunity for Stanisław Wyspiański and other art nouveau artists to redecorate, and the result is fabulous. The walls are covered in a riot of colors, with geometric shapes and plants that evoke the redone stained glass windows.

One of the benefits of its redecoration is that is is much more unified in design than some other churches, which slowly accumulated more and more paintings, statues, monuments and more over the centuries.
The church’s side chapel isn’t as spectacular, although it does have a good series of paintings depicting the Stations of the Cross and an allegedly miraculous replica of the Shroud of Turin, although given its location on the side I can’t help but assume that the clerics running the church aren’t convinced of its miraculous nature.
It can be slightly awkward visiting churches in Poland, because Polish churches frequently have worshipers praying in them outside of Mass, and I assume that tourists rubbernecking at stained glass does not improve the praying experience. (Although I do wonder why someone would choose to pray in a church frequented by tourists instead of one that is not. It seems like it would be distracting.)
Of course, many museums contain pieces of art that were originally in houses of worship, and there folks don’t need to worry about disturbing worshippers. Unless, of course, there are worshippers in the museum.
The Bangkok National Museum contains statues that were once in temples in Thailand, where worshippers made offerings to them. But now that the statues are in the museum, some of the worshippers have followed them to the museum. It’s unusual by Western standards, but in a country where businesses, homes, and even classrooms have small houses for local spirits, it makes some sense.