Marvel at the size of Cathedral of the Assumption. Go to a service if you can or simply take a seat and enjoy the peace and quiet.
You'll be spoiled for sightseeing choice in Varazdin. Cathedral of the Assumption is a great place to begin here, but there are many other worthy places to consider. Town Hall, Varazdin Cemetery, and Gallery of Old and New Masters are near the principal church and will fill in your day nicely.