We did something like Cookie's suggestion, but used half-size poster board and post-its. Each of us - myself and both kids - got personal boards, categories that made sense to us, and our own post-its. My kids actually saw me making one for myself and liked the idea so much they made their own.
My categories are defined by the kind of time I have: half a day, five minutes, while in the car, with friends, alone, etc. I think my younger one made categories of days of the week and chose one fun activity to plan on for each day. They don't always happen, but I get fewer complaints of "nothing to do."
To make it fun, we chose post-its that were pretty. These are nicer than written lists because they are more changeable, but better than dry-erase because they last longer and are nicer looking.
