From Lucerne there is a daily bus to the Thermal Spa - Krapinske Toplice, the bus drives via Zagreb, where passengers has to change bus. The travel time fro the 920 km long bus route between Lucerne and Krapinske Toplice is about 13 hours. The bus arrives in Krapinske in the morning so you will have a full day to enjoy there. If you are travelling back to Lucerne you should buy a discounted return ticket, prices and route information you can see below, returning timetable here.