OK, well for the fireplace thing, what I do, is this.
First, I start by making sure they know what hot means. This starts at dinner. Whenever I put food in front of him that is hot, I say, "hot" and point to the food. Of course, when he goes to touch it, he notices that it is not comfortably warm. Though I never serve him hot hot hot food, I do serve it hot.
OK, our fireplace is also difficult to cut off from his reach, especially since one side of it is right next to a doorway and there is no room for a gate of any kind. So, here is what I did...
Simon NO
and got up and moved him away...
Repeat until he gets it. What he needed to understand is that when Mommy says NO, mommy is going to MAKE that NO happen. No matter how many times or in how many ways he tried to get past it, Mommy made the NO happen. So, eventually (mind you after a couple weeks of lessening incidents so for instance on day one there were 20 no's and on day 14 about 2 no's) eventually, he got the picture and realized that no matter how he does it, the fireplace is a NO.
So, when I first lit a fire in it, he came close, I did two things. First, I said "hot" and he of course leaned a little closer to feel the heat. He wouldn't go any closer because he knew that the fireplace was a NO.
Now, I know this isn't a quick fix, but, in the long run, it was worth it. Of course, I have two older children who helped. I would say NO and sometimes bubby would move him away. Sometimes Sissy, so he knows that if ANYONE is in the room, they will enforce the NO.
That is another reason why I love GBD, the other children can HELP teach the younger children. There is no fear of them trying to "discipline" in an inappropriate way. For instance.....
DD (4yrs old) spilled cereal all over the floor and wanted to just eat her cereal in her bowl and leave the stuff on the floor. DS (6yrs. old)told her, Sissy, you know you need to clean up the messes you make. And then he offered to help her. I was in the bathroom and heard this happening and just LOVED that even my son can discipline (teach) my younger two.