One thing that now also has to fit in my bag is my "house book." I was out and about to many times realizing that we needed new air filters, but I couldn't remember the size or loving a pillow for the bedroom but unsure of how it matched our paint color.
I found a small notebook that closes with a snap and has a pretty tough cover. In it I make notes about anything house related that I may need to remember on the fly. I have a list of the size/type of air, fridge and vacuum cleaner filters; measurements of spaces in the house that I need to fill with furniture, and all of the paint chips or fabric swatches (if I have them) that adorn the house.
By keeping this in my bag I can tell you:
1-The air filters for our house are 20x20x1.
2-I am on the hunt for a buffet for our dining room that can't be more than 16 in deep.
3-The color we painted our master bathroom (almost 3 years ago) is Olympic Silver Blueberry from Lowe's.