We've got fireplaces in our house where we live, and I've never needed a baby gate or anything to block it. My son is now 30 mos. and what I started doing around your son's age is, I would sit down in front of the fireplace on the rug and ask my son to sit with me. I'd sit and tell him that a fireplace can be dangerous, and it isn't safe to play any further than the edge of the rug. We do use our fireplaces during the winter, and over time he started to understand that there are limits and safe zones. He knows that when he's in the kitchen and I'm cooking he can sit and watch from a safe distance.
Really, kids just need to have things explained to them. They learn from our example and understand more then most give them credit.
With my little guy, timeouts don't work. He's a sitter, so he could sit for hours on his time out mat and it doesn't phase him. So, we do toy timeouts and he loses 'tv time' if he does something he knows is wrong after being told not to. He only watches 3 shows in a whole day, so it seems like a lot to him. Our toddlers don't necessarily mean to do wrong, or do things to spite us, they really just want attention and reaction for what they have done...wrong or right, good or bad.
Try giving him some structure to his day. When my son is home with my Mom, she does 'learning time' and 'fun time' so, that he has some sort of structure to his time. During 'learning time' they do puzzles, work on shapes and colors, and other fun projects, and during 'fun time' they paint and do crafts. It gives him something to look forward to. Also, we started doing classes and activities outside the house. Gives me and him time to breath, and shake loose from all the time in the house (or at work for me).
Just be firm, be patient but, above all being loving.
Good Luck, and don't forget to have fun.