I'm one of the ones who considered Zagreb kind of "ho-hum." I spent a few nights there back in 2009 at the beginning of my first Croatia trip, and I was done with the main sites in about half a day (I was picking up a car there - it's a convenient place to do that or return one). I like cities, too, but for some reason I didn't find Zagreb very enchanting. I would not go out of my way to see Zagreb myself unless passing through. On my last trip to Croatia in 2015, I flew home via Zagreb (dropping off my car there) and chose not to spend any time there, even though I like to revisit places I've enjoyed (I've been to Ljubljana three times now - love it!) If I had extra time, I'd probably push on to Plitvice Lakes National Park (which I found amazing) about two hours from Zagreb. Or, I'd see other spots in Slovenia. But again, who knows? You might wind up liking it more than I did.
As a compromise, if you have two nights, you could spend them in Zagreb and do a day trip to Plitvice (if that interests you at all) and spend your evenings in Zagreb. There is bus service from the park to/from Zagreb. Plitvice can easily be seen in 5-6 hours.
Most of Slovenia is not very touristy by the way, at least by English native-speaking tourists like Americans. That's one reason I have found it so delightful compared to, say, Italy, which is great in parts but also overrun with tourists in too many places. (Plitvice is quite touristy however.)