I might have missed some other sources and I probably have the order wrong, but I came up with this list 1st thing as I woke up this morning when my thinking is usually clearer than the rest of the day.

Sources of Water that Add Humidity to a House:

9. furniture, walls, all house – it wicks up humidity from air and then wicks it back to air – works as temporary storage of water
8. plants
7. “bowls” of water such as the toilet, the dirt in a potted plants, a glass of water..
6. doing laundry
5. from outdoors when weather permits
4. people – they eat, drink, and then breathe and sweat it back out
3. taking showers
2. cooking
1. Humidifier!

(Thanks, Josh, for adding #1!)