Here are some icons I've created for use with the excellent mobile phone application, Aquacalendar from PocketTorch (available to buy from here - includes a free two week trial period). There are three sizes of icons - any version should work fine but the smallest set (12x12) are the latest versions, include more icons and should work best with the Series 80 version of AquaCalendar (as they won't overlap in certain views).
If you want to make your own icons, Hedlasse has a website that contains a kit for making icons. This kit was designed to create icons for the UIQ version of AquaCalendar but will still work on the Series 80, however I'd recomend making any new icons for Series 80 at a resolution of 12x12. You should also (ideally) incorporate a greyscale layer for each icon so it contains transparency. The process for this is explained here.
You can download my 12x15 icons (the first versions I did) here
You can download my 12x13 icons (better for series 80 version of AquaCalendar) here
You can download my 12x12 icons (best for series 80 version of AquaCalendar) here