Take him to the park
That's quite an obvious one, but you simply can't go wrong with visiting one of many parks Ljubljana has to offer. The most popular park is probably the well-known Tivoli, but there are actual dog parks in Ljubljana too, where dogs can run around without a leash. A fenced area, of course, where you can sit and relax on a bench, while your dog plays with his canine friends. There's one dog park in Koseze, at Štepanjsko naselje neighbourhood and one at The Northern City Park. My favorite though, is the dog park at Šmartinski park. It's the largest and there's a car park right nearby. Don't forget to pick up any poops after him. Don't worry, poop happens. That's why there are many doggy "poop stations" in our parks, which offer you bags and a place to discard them.
