I married an engineer. In doing so, everything we do for our children comes back to the same question, "Does it scale?" With two of them, everything we do is about the road of least resistance... for us and the babies. The plan was to teach them independence from the very beginning and so we adopted a lot of Montessori ideas from bedroom set ups to baby proofing.
Independence scales. It's a lot of upfront work, but in the long run eases a lot of stress. Part of building independence is minimizing the n.o. word. You want to set your house up so that you say no less and encourage exploration more. The real power of this is that when you do say no, you mean it and it carries weight. They listen, remember, and test you less.
The other part of building independence is fostering curiosity. Curiosity is tough if you're having NO yelled at you at every turn from cupboards to everything they find interesting locked or off limits. It came down to real safety.
Instead of locking cupboards, we put all of their cups, bowls, and Tupperware at their level and anything sharp and breakable way up high. Don't get me wrong, I found them many times climbing into drawers and having to stop them, but that was only one, "no", instead of many more.
We did have gates at the top and bottom of steps. This was non negotiable, and when we went up to bed or came down in the morning we took the time to teach them how to do it.
We didn't lock any doors. We even kept some of their toys in the cupboard under the stairs where they'd go open and close the cupboard and take out their own toys. They loved tearing that closet apart. Taking out all our shoes and trying to put on our coats. Annoying to have to put back together? Yes, but it kept them entertained for more than the standard 10 minutes. You exert more energy in trying to stop them rather then guiding them. Leave them the path you want them to find rather than the path you have to prevent.
Non Negotiable - Meaning If something goes wrong it can't be undone
Stairs. Block em. The minute baby can roll over on their own crawling is coming right behind it. Or if you're using baby walker you'll want to make sure baby can't fall down them.
Furniture. Bracket bookshelves it to the wall. Especially tall heavy ones.
Windows. Specifically low windows on the second story they can climb on. For horizontal sliding windows you'll want to drop a poll and for vertical you'll want to place stoppers so they can't open it more then a few inches.
Electric Sockets. Plug them up.