It's often interesting to look into different belief systems. An outing to Church Of St Leonard is a good way to deepen your knowledge of one of the local religions and cultures.
You'll have no end of sightseeing choices when in Novo Mesto. Church Of St Leonard is a great place to start here, but there are loads of other worthwhile attractions to appreciate. Struga Castle, Otocec Castle Park, and Church of St John the Baptist are close to the house of prayer and will fill in your day nicely.