Personally I like to put mt characters in different scenarios, an adventure, a romance, a mystery etc. If I like any of the scenarios and how my characters look/behave, I try to extend it and make it into a working plot
I also day dream a lot so if I have a weird day dream I like, I'll put my characters into it and see where it goes
For example I was thinking about writing romance and found a story in my mind about a character running into another characters room to confess but not understanding how to confess their feelings. I really liked the way o worked ut un my mind so I started it on paper and ended up liking the way the story started working