Ljubljana and Slovenia in general don’t have rigid rules when it comes to tipping. Tips are not obligatory but appreciated. The locals we talked to say they usually just round up the bill and tip if they enjoyed the service.
At restaurants, the service charge is already included in the bill. But you can still a tip if the service is exceptional.
On paid small group tours, you can tip 5-10% of the cost. At FREE walking tours, most participants leave 5€ if it’s a big group.