I recently broke down and bought some Field Notes (they were on sale, of course). These are pocket sized (3.5 X 5.5 in) notebooks. These durable little 48-page books have numerous uses - jotting down notes, tracking yard sale finds and how much I sell them for, Sunday school class ideas, and calorie counting to name a few. Here's the one I carry daily on the left. I've used a Palm Pilot, smartphones, an iPhone but, for me, nothing beats good ol' paper and pencil. You get three of these for $9.95 plus shipping.
Ask me about mine if you see me; I should have one on me.