Where are you that you need a room key to use the elevator?  I think I've been in at least 1000 hotels in over 12 countries and never needed a room key to use the elevator.  Not saying there isn't one out there but...

Nothing to worry about...

First make sure elevator requires a key card to access your floor. Many but not all do.

Tell her to dress appropriately which she probably does already and walk into the elevator towards the center with a group of other hotel guests. Once inside, ask her to politely ask another guest to enter the floor number for her. Many guest will do it without thinking twice about it.

You only need to swipe a card once to activate the control panel, then you can enter as many floors as you like unless you are a Mr. big shot and have a specially coded card for your floor.

Remember hotels don't really want to piss of guests unnecessarily and make things inconvenient for them.

Depending upon whether she is coming over to your room, or vice versa.

Regarding the former, I assume you are having a convention of some kind, so it's  big hotel with function rooms, etc.  Have her meet you on one floor with the function rooms where there won't be any people around at that time.  Always ask for two keys, and you can give one to her, then head up and have her head up a few minutes later.

If the hotel does't have such facilities, then use a store or coffee shop within walking distance for this purpose.

If you are coming to see her, just use the converse procedure where she gets two keys and meets you.  She might also find a place somewhere in the lobby where she can leave an envelope with a key in it beforehand and direct you to where it is so she never has to come downstairs.  For example, she could place it in a magazine and leave the magazine on one of the end tables in the lobby.   A phone book at a pay phone would also work.  

Another way is to have her come down the elevator, but never get off same.  She describes what she is wearing (and you likewise) and when you see a car come down with her on it, you just hop on and both ride back up.

Personally, I would never leave a room key somewhere...   I think the coffee shop handoff is easy enough.  Agree key card access is a PITA.  
Hey, guys, stratigy for least conspicious room choice?  Maybe a different floor?!

Look. I'm not married. Most people know what I do because I don't give a fuck what they think.

I have no problem, what-so-fucking-ever, taking the elevator down, seeing the lady du jour, grab her hand, and take her back up before the doors close.

I realize there's cameras everywhere. I don't give a fuck.

I give you your donation behind closed doors, and the only thing I can think of that might raise a flag is if we start to strip in the elevator and all the way to the room.

Don't get me wrong. I've grabbed and groped some of them on the way back up, and it was reciprocated, so I know it's cool.  

Thanks to fire regulations, once you are in the stairwell you generally have access to any floor.  Enter the elevator with other people in the lobby, ride to wherever the elevator goes, and then take the stairwell down to the appropriate floor.  Many times, as others have said, you only need the key card to activate the panel.  Once someone uses the card, you can push any button.  Sometimes, the card activates buttons for the appropriate floor and all floors lower.  You can test all this stuff ahead of time and tell her what she can do.

Honestly, most women have dealt with this before and handle it fine.  A beautiful lady can flash a smile and pretty much get whatever she wants if she's smart.

Or just quite worrying and meet her in the lobby.  Discretely go to your room.  Most people are not going to suspect anything.  And those that do, probably do only because they know from personal experience how these things work.