You will be hard put NOT to meet actors in LA. Anyone waiting on you in a bar, cafe, or restaurant is likely to be an actor, and quite possibly one who's working too. Or at least as much as most.
You'll undoubtedly meet actors in most coffee shops, but bear in mind that resume-inflation is not unusual in this town and some of the people who have the most time to chat with you will probably be the ones doing the least.
If you want to meet relatively serious actors, a surer bet would be to audit as many classes as you can and chat with people during the breaks. Also, check Backstage and the LA Weekly for well-reviewed productions and try to go see a few, maybe talk with the actors after.
As far as well-known actors go, you generally run into them where you least expect it. Personally, I've met a number of very well-known people in cafes and at art openings, but you'll find people in gym classes (often teaching...), supermarkets, on buses, etc. as well.
For generally nice places to be (with no more or less posers than anywhere else, I'd guess), I might mention Priscilla's in Toluca Lake (right near Warner Brothers), Indie Coffee (by the Pitfire at Magnolia and Lankershim in North Hollywood), Aroma Cafe (on Tujunga Avenue in Studio City, right across from Vitello's the restaurant where Robert Blake didn't kill his wife), the Starbucks at Beverly Glen Center, the Urth Cafes on Melrose and on Beverly Drive, the Coral Cafe in Brentwood, Mani's on Fairfax, a few places in the Farmer's Market (off Fairfax)...
Come to think of it, try cruising the weekend farmer markets, in Hollywood, Studio City, etc. Also a fun mix of people.
Etc. There are actors everywhere.
The one other thing I'd warn you about is that you'll meet lots of "producers" out here (not to mention a few directors, etc.) Just be aware that that can mean a host of different things - even when it's real.
In other words, talk to people you really connect with, not people who seem like they might be useful somehow. Because there's a lot of people in this town trying to look way more useful than they are.