I love Post-It notes! I have drawers full of them. I use them pretty much daily. I have a rubber-stamp from Etsy (seller is CoolJapanStamp; see here for shop) of a To-Do checklist that fits perfectly on the 3" square Post-Its, giving me a portable To-Do list that can get moved from diary to desk to wherever.
|Post-It notes plus To-Do stamp|