Most language teachers will agree that teaching the preterite vs imperfect is not a fun task. In English, it's something we do naturally, but in another language, it's a whole other story.
I've always given them a list of common "trigger words" to watch out for in regards of passe compose vs imparfait, but that generally is more useful only in fill in the blank activities.
I tried something super simple today but super effective. As a warmup, I projected a random photo I found on Google on the board. In this photo is an older couple eating breakfast, and the clock is even behind them. I informed the class we would be writing a story, but every story needs its background. We quickly review that the background/setting of a story is the imparfait, so I instruct them to come up with a few settings of what they see in the photo as the background of the story. They did this individually. After a few minutes, we came together and started the story. I even let them pick a name for the couple.
Once we established the setting, I let them pick groups of two or three. I showed them the end photo of the story and told them their task was to continue the story to wind up at that end. I reminded them that they needed to have the metaphorical "bomb" go off (the interrupting action as they know it... more to come on that later) and switch to passe compose.
Each group presented their story, and they were all very different! It was fun to see what they came up with. It was also great practice for them as their presentational assessment which would be to write a short story using their vocabulary as well as their knowledge of imparfait and passe compose.