We did a couple things that ended up being useful, and others that are (so far) unnecessary. Caveat being I took care of a lot of pediatric trauma so my experience is skewed significantly.
1. Covers on the electrical sockets (unless you live in Europe, apparently). Our son picked at them for a day and then gave up. I've never seen a major electrical burn from 110V, even in children, so it's probably low yield. However, covers are cheap. 220V can cause significant injuries and those outlets should be covered or deactivated if not used.
2. Straps for TVs, monitors, bookcases, stand-alone cabinets, and anything else heavy that can fall on the child. Lots of severe trauma from kids pulling TVs onto themselves. It is best to screw the straps into the wall studs (not sheetrock).
3. Hot water - turn your tap to max heat. If it hurts to keep your hand under it for more than a second or two, it can cause severe burns. Go to the water heater and turn it down until it's safe. Children burn easier than adults, especially on their face and palms.
4. Anything that gets hot and can burn should be kept well away from kids (tea kettles, room heaters, hot water on stove, deep fryers). Metal surfaces usually don't cause major burns because the kid will reflexively pull their hand away. More important is keeping them away from hot water or oil. They cause deep burns since it's harder for the kid to "escape" the heat source.
5. Any cleaning agents - keep them on high shelves rather than under the sink cabinets. Your child will figure out how to open those door locks eventually. Don't make it possible for them to try to drink the Draino.
6. Covers for knobs on the stove. Prevents your house from filling with combustible gas.
7. Locks for toilet seats. Toddlers can fall into toilets and drown. Also, never leave them unsupervised near any tub, bucket, etc filled with water.
8. Any power tool, or really any tool that has moving parts should be locked away. Finger amputations require very little force. Hand tools are less dangerous but can still hurt if used as a toy.
9. Gates at top and bottom of stairs. Falls from couches, chairs, etc are unlikely to cause significant injury. Falls from stairs are much higher risk of neck or head injury. Also, don't leave ladders around. If for some reason you have a permanent ladder, remove the first few rungs so they can't climb up.
10. Play equipment outdoors should be properly secured to the ground. The most common serious injury are from these things are swings that are not secured and tip over.
The reality is a properly supervised toddler is unlikely to seriously injure themselves, but it's impractical to watch them every waking moment.