We did an exercise in my writing class which was to just start a series of sentences beginning with the words "I remember", e.g. "I remember my first day of school", "I remember I sat on the red table". One thought will normally organically lead to another, and you'll develop quite a list over the 5-10 minutes you do it for. As a side note, I also find this technique really useful for character development, if you do the same exercise but from one of your character's POV (the first sentence for one of them was "I remember the day my parents sold me").