I would definitely choose the sad event, or a regrettable one that's pretty sad in its own way. I wouldn't want to change a historical even because that could also mess things up. other than that, I wouldn't change a lot of things because they are fine the way they are, and i don't want to ruin what already is.

I chose "Make someone happy by changing something in their past", as I can't really think of another reason to use one, other than re-watch moments in time that you wish you could see again..

I would choose to change a sad event in my life. I wouldn't want to change a historical event, because who knows what that could do. It could change everything! In general I don't think I really need to change anything, because I pretty much like the life I'm leading. And also, if we did change something, even something very small, you never know who or what that could effect and if it'd be for the better or worse.

I would change a sad event in my life.

Otherwise I would probably use it to live twice as long, and do more/less (depending on your perspective) on the things I like!

Save someone. I was only 3 years old when my grandfather died, but my mother and my grandmother went through a really rough time and they still suffer. I could prevent him of making effort (he had a weak heart) and maybe he would have lived longer...
Also, I would have liked to start guitar lessons, foreign language courses and other productive stuff at a younger age.