Some additional ideas:
GPS tracking (don't yet have a GPS phone, but will soon)
Software that will give me heavy-handed reminders to do certain chores or tasks.
Motion sensors that will graph amount of motion in my room over time.
Sound sensors that record the level of noise over time.
I'm interested in any kind of quirky, creative, practical, or otherwise interesting trackable data. It needs to be trackable in a somewhat passive sense without much input from me.