Recommendations for Private or Small group tour from Zagreb to Lake Bled and Ljubljana

I have no tour companies to recommend, but there's both train and bus service between Zagreb and Ljubljana. You could get there in your own and hook up with a guide if there aren't suitable half-day tours offered in the afternoon. Lake Bled is close enough to Ljubljana that using a taxi in one or both directions might not be too costly. There's bus service, but trying to do Ljubljana and Lake Bled in a single day--and from Zagreb--means every minute will count.

Personally, I'd choose just Ljubljana since you'll have substantially less than a full day. And I'd try to find a walking tour, then just wander on my own. It's a really nice city.

Lake Bled is pretty, but it's very tiny. With the time you'll have available, you won't be able to do any walking, I don't think, and you won't have time for the castle. I'm not sure you'll manage the boat trip out to the church, either. I'm afraid you'll burn a fair amount of your time basically traveling to see the (very lovely) view that's on so many travel posters.

This is going back 4 years, but my wife and I used The Guide (https://www.theguide.hr/category/our-tours/) for a day trip to Lake Bled and Ljubljana from Zagreb. It worked out well for us.

I wouldn't recommend doing both on the same day trip, it's quite rushed and if the border crossing is congested you can lose a couple of hours there alone.