Elijah didn't crawl until she was a year old, so she didn't move if I left the room for whatever reason
When she did finally start moving I had a playpen that was basically 6 plastic sides with activities on 3 of them, which joined together. If I was in the room it was left open, but if I needed to go upstairs, I put her in it and closed it up. Same with Rheiya, but we were living in a flat by that time so it wasn't closed that much as she just followed me
With Lucas I just left him! The lounge was completely safe, worst he could do was pull out all the CDs while I nipped to the loo.
With Jaidyn I had one of those baby activity centres, so I just dumped her in that if I needed to go upstairs. She loved that and once in it would stay in for a good half hour or so (screaming blue murder if you tried to remove her before she was ready!) which was handy for the dreaded bed changing days