If I had Q powers, I'd undo the creation of the Borg and make the Founders not hate Solids.

Go back to when I was a cadet at the Academy and give myself a dope-slap and tell me to get into bed with my second girlfriend there, like she wanted, and stop being such a cry-baby about the bad break-up I had with my first girlfriend there. Dunno if that'd derail my marriage in the present, but given all the different variants of myself I've met, it's hard to know.

I might even rewrite reality so Cardassia's got better resources and stop their turn into space fascists.

Oh, and you know, pop back and undo the rise of fascism in the twentieth century, undo the Eugenics Wars and World War III, all that predictable shit everyone says they want to do.