Very interesting YM...I don't think I was on this forum when you posted this stuff before. I'd really be interested, as would Rob, in hearing about more.
All good suggestions. And don't forget (assuming that it's possible to drive close enough to see it): the famous "Stahl House", featured in multiple "Columbo" episodes including "Prescription: Murder," "Murder By The Book" and "The Most Cricial Game". See:
http://www.columbo-site.freeuk.com/flashback.htm
If you're willing to drive out of town, a memorable stop would be the Palm Springs Aerial Tramway, from "Short Fuse":
http://www.pstramway.com/
And by all means, no Columbo fan's visit to LA is complete without a stop at Barney's Beanery!!!... portrayed literally in "Ransom For A Dead Man" and in fictional form in several later episodes. See: