I lived in Leuven for over a year, and despite not being religious, Sint-Pieterskerk was one of my favorite places to go for a quiet moment. I find it to be a very unique church as most of the stone work is white and all the windows are wavy-clear glass, so it always very well-lit in the daytime, unlike many of the churches with light-impeding stained glass (though gorgeous in its own right, of course!). You can pay a few € to go through the back which is a bit of a museum with a few very nice paintings and you can go see the crypt. If you're interested, it's worth it - but I find the church itself the thing to see, so just enter from the bustling streets of the Grote Mark and Bondganotenlaan and enjoy the serenity!
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