depending on where you get the room there are a few things you can do.

On screen check out, viola no dealing withe frnt desk

Get the room though Priceline etc.. so its prepaid, no need to check out.

leave your key and let them charge your card the next day.

If you have to check out at the desk, just say you had a family emergency and need to check out right now if someone asks. Sometimes they ask because they don't want to think you are checking out so early because the hotel did something wrong.


Jenny

You don't actually have to "check-out".  The hotel already has either your credit card or your cash payment for the room. Any additional charges will go to the credit card that you have on file.  You can leave anytime and your folio will be settled automatically.

You can just drive away after you are done using the room.

I've rented hundreds of hotel rooms, I've never had to check out, I just leave. This is possible unless you pay cash in which case they will likely make you leave some sort of a deposit for incidentals that you have to collect at check out.

FYI: If you're renting hotels to see escorts it is NOT a good idea to pay cash as this can put you on the radar of LE who sometimes ask hotels for a list of cash paying customers as that is typically the M.O of drug dealers, escorts and other "criminal elements". Just don't do it.

The guy at the front desk gave me a discounted half day rate.

There's nothing illegal about meeting up and having sex in a hotel room for a couple hours...

as a matter of fact, I'm quite sure it happens all the time!

One tip I read about, do not leave the lady with your room...  one used the room for calls all day, ran up a bill from the mini-bar, etc.

Often a lady will arrange the room (making it an "InCall".  If it's a room just for your visit, you could leave her reimbursement for the room.  
skb

No need to check out, just leave. That's what I've been doing for years with hotels. Just leave when your ready, and leave the keys in the room. No problems...

I usually don't get the room, but on a few occassions when I did, I just went to a motel that accepted cash and did not require ID (i.e., not a 5 or 4 star, but decent), signed in under a false name and fictitious address, paid upfront in full, then when done, just left the key on the bed and left.  If a formal checkout is requested, just call the motel and let them know you left the key on the bed - as long as you are paid in full, they don't care.  And, as another noted above, the clerks know this happens all the time - they don't know if your with a girlfriend, provider or whatever, just so long as you pay and don't make a mess or cause a disturbance.