I just walked in during the middle of the day. There was nobody at the front I didn't see a single employee. I decided to just go for it and walked down the hallway until I found the room. There was a key card machine for the door but it wasn't shut all the way. I walked in and nobody was there. I used the treadmill and bench press and enjoyed having the room to myself.

Edit: I don't know what some of the comments are arguing but I'm not claiming this was a "spur of the moment" thing. I tried this at another hotel beforehand and it didn't work. I made a plan to try several hotels to see if I could pull this off and I got lucky at the Holiday Inn again.