Although falling down the hill in "The Greenhouse Jungle" wasn't really an ad lib (more like an accident), the following dialogue between Columbo and Sgt. Wilson was a pretty good ad lib.

Columbo: "That's a little steep. But I tell ya' it was the quickest way down."
Wilson: "Oh yes sir, by far".

And then Wilson helps him up and brushes him off and Columbo gives him a funny look.

I think all ad-libbed.

While there are some obvious ones, much of the genius of the show and Falk is that you can't often tell when he does it or not. Bochco explains it so well here at the 7-minute mark: https://www.youtube.com/watch?v=O2wOnLS7lqc
I'll keep looking for them though...

Almost every actor in Last Salute For The Commodore seems to be doing a little.