Los Angeles
is the Century Plaza Hotel in Century City. No elevator key cards required, day or night. No security guards checking. The lobby is very big and very busy, pretty much all the time, so there's no issue with a hot young girl being noticed. There are hot young girls all over the place! There's a bar right in the middle of the lobby and another bar off to the right (X Bar), so you have options for meeting there, if you prefer.
All in all, it's a perfect hotel for hobbying. It's somewhat expensive - around $240 per night - but if you use Hotels.com and choose the Hotwire last-minute deals (blind booking), you can usually get a room for $80 to $90 below the normal rate. Just use zip code "90067" to search. Select "4-star hotels" and you will almost always get the Century Plaza or the InterContinental. The InterContinental isn't quite as good for hobbying, though, because the lobby is often a ghost town and a hot young girl will definitely be noticed there. Not that they would do anything about it, but she WILL stick out like a sore thumb. I can't remember if they require key cards in the elevators, but I think they do.
Also, the Century Plaza is old, so it has very large rooms. Plenty of room to play all kinds of fun games with your lady! 
Hang out in the hotel lobby and try to guess who's hobbying, who's on vacation, and who's there for a convention? Or who's doing all of the above?!
Doubtful. Too many easier ways to bust people if Vice needs to fill their quota for the week.