Well, I have fairly responsible kids, but some of the fun things they have done include:
Climbing the 8 ft, solid oak bookcase. Yeah, we bolted it to the wall after that.
Running into the back yard because he didn't want to go out with us, my son tripped and face-planted on concrete stairs. It busted his lip, tore his mouth up, and caused one tooth in front to turn grey because the nerve ending died. Thank goodness it was a baby tooth! And the worst part was that he had to go out with us any way -- to the emergency room.
Climbed the metal T-bars that hold the clothes lines, then stood up on the top. You can't yell when that happens, because you might startle them, but your heart stops while you wait to see if they're going to fall. Mine didn't.
Hiding in a cabinet/closet/foot locker and discovering they can't get out. Shortly after that, there were no hasps on the foot lockers in my house.
My younger son wins the accident award, and while he did it in gym, it could be adapted to happen outside or in the house. He was rollerblading, tried to stop too fast, and did that lovely windmill thing as he fell, twisting his leg and then landing on it. So he ended up with a spiral fracture caused by his butt. That story is going in the arsenal for when he starts dating.
Never underestimate the power of sporting equipment as means for a child to hurt herself and/or others. Hockey sticks, bats, golf clubs, lacross sticks, etc. are good candidates as you can thwack yourself, trip over them, swing them wide enough to send things hurtling towards your head, etc.
Throwing balls inside is a good way to make sure something breaks and subsequently cuts you.
And the trampoline is just an emergency room visit waiting to happen. But we still have one....