Calendar (with large writing area per day or tiny hand-writing - Lol) and/or desk diary are, it seems to me, quite decent options. However, I would also say that a spreadsheet would be more flexible if one can be bothered to set up.
I haven't done this yet, but if one put plant names (randomly) vertically in one column and then had one's key planting/harvesting factors in other columns and work out a consistent code for entries, it would be possible to "sort" your columns according to what you want to know, e.g. what to do in each week of the year and then maybe whether it went well or needs to be reconsidered for the coming year. (Much as suggested by Q)
Examples (slashes = columns): Plant (name) / Seed (S) or Plug (P) - could be 2 separate columns / Week of sowing or planting - 1 to 52) / Indoor or Outdoor (I or O) / Planting out week for indoor propagated stock / Take (Good = G, Iffy = I, Poor/failed/slugs! = P / First harvest Week / Last harvest Week / Harvest result (Good = G, Iffy = I, Poor = P / Comment-action for next year.
If repeat sowing/planting over a period, add a line entry for each repeat to enable a direct comparison between the success of successive plantings.
Of course, depends on how happy one is with spreadsheets and ways of sorting their entries by various criteria.
(I use free version of the LibreOffice suite of office app's - fully functional and, as far as I can tell, pretty-much equal to MS Office. If you have not come across it, it has Writer, Calc, Database and other MS Office-like app's and is compatible with MS Office - it can read and can be simply tweaked to produce/save files in MS Office file formats.)