Hot Places in LA
Instead of Koi, I'd say Katana for better food.
I always see celebs at Giorgio Baldi.
Nine-Thirty at the W.
The Ivy is always "hot" for celebs, and yes it's expensive, but for a great grilled veggie salad and crab cakes and tarte tatin... it's good.
You might also look into...
Hidden on Main
As others have, I'll mention both proper restaurants and bars.
In Beverly Hills / Sunset Strip, try Blue on Blue at the Avalon Hotel or Skybar at the Mondrian... both very cool poolside bars, but on completely different scales. (Skybar is massive and downright overwhelmingly chic; Blue on Blue is much more intimate and low-key)
In Los Feliz / Silver Lake, Tantra and Malo are both fun spaces with good food and drink.
Then in West Hollywood, pretty much any hip place on 3rd or Beverly or Melrose -- there are a ridiculous number, as the majority of the top-rated restaurants in L.A. are all clustered here. The Little Door is a beautiful place with amazing food; AOC is a favorite of the small-platers (not enough veg options for me, but the meat-eaters all love it); Dolce is a cool place with good food and a flaming bar; Xiomara is the king of mojitos and makes you feel like you're Havana royalty in the 1950s; Providence is regarded by many as L.A.'s best restaurant, period.
Finally, in Hollywood proper, if you just want a fun place to go mingle, the Cat 'n Fiddle is an enjoyable outdoor booze-filled zoo.
I would suggest the Tower Bar at the Argyle-"cool" in an old Hollywood way, the food is pretty good and the view of the city lights is nice. I saw many celebs when I was there. Also, Koi on La Cienega might fit the bill. Boa in Santa Monica or the Lodge on La Cienega are steak houses with "atmosphere."
You do realize how vague "cool", "hot" and "atmosphere" are, right? I'm going to assume that this translates to a laid-back vibe ("cool"), upscale clientele ("hot") with some people watching thrown in ("atmosphere").
The hottest ticket in town is definitely Osteria Mozza. Plenty of celebs frequent the place if that's the type of scene you're looking for, definitely abuzz with activity. And to top it off, the food's pretty good, too.
In that neighborhood, also, is AOC -- definitely a great people watching spot, certainly upscale but with it being a wine bar, it's a bit more low key than a traditional dinner-menu place.