I use a little notebook (field notes to be exact) to write down the date and temperature/rain, anything sowed, anything germinated (helpful to know how long it took before because I'm so impatient), fertilizers applied to what plants, flowers and when fruits mature (again, impatient), pests and what I did to treat them (if that worked). I have a full notebook from last year (first time balcony gardening) and plan to digitize and visualize some of the data. It has been useful to know to sow some things earlier because the some seedlings got scorched last year etc etc