The simplest of things can be the most effective when it comes to planning.
Two things I wouldn't be without for any sort of planning are my diary and a notebook.
The notebook is small enough to fit into a bag (or pocket) when shopping. Large enough contain all the lists I make and notes of my ideas, but not so many pages that some are wasted.
The diary is quite chunky and will take take all the information I need on a day to day basis throughout the year and with a pocket at the back to take loose papers, tickets, etc.