What’s the cheapest thing you’ve done that measurably changed a metric?

Mine today: put the audio track of a carousel post as one of the items inside the carousel, rather than picking a generic background song. If you hear a song and want to know which one it is, you swipe until you find it. Costs nothing, takes no extra work, and it turns passive audio into a reason to keep going. I'm a solo founder with basically no audience, so most of what I can do has to be free. Which turns out to be a decent constraint — it forces you to find the mechanics rather than buy reach. What's yours? Small change, real difference.