It would probably make more sense to deal with the problem areas than try to fence them all off. Kids want to wander around and interact with you. A playpen is fine as a place to have the kid while you go to the bathroom or something, but you don't want them just in there all the time. If you have a glass coffee table, or something, I would just get rid of it. Even when kids get older, there's still a lot of running around. It should be possible to make an entertainment center safe. You just need to anchor the monitor into the wall if its reachable, get baby locks for any cabinets and get cord/outlet protectors if needed. For the fireplace, we got some of those interlocking modular gates. Even if there's nowhere to actually anchor them to the wall, they still will keep the kid from being able to just walk right up to it. Not everything needs to be completely baby proofed in the sense that there's no way they could get into something they shouldn't. You just want to feel like you don't need to be trailing along a foot behind them at all times and never taking your eyes off them.
Thank you for saying this ocelot1066! Exchanging pieces of furniture temporarily is a great way to baby proof. Op let the baby explore their surroundings, safely. Teach them what they can and cannot touch. Model for them what is safe movement (ie how to get off of the couch safely etc). Treat babe like the little human they are.. its amazing how much they learn through mimicking!