Do NOT store them in the watercolor sets unless everything is totally dry first. The best palettes I've found are the stiff plastic disposable plates. They can be washed and reused many times. Lately, I've had trouble finding white ones however and they must be white. Styrofoam plates or paper plates or regular white plates all work too.
Kneaded erasers are cheap and nice to have for all drawing projects.
They're also a great outlet for excess nervous energy. I personally use an old diaper for this purpose and kids find that hilarious. It's better to share big containers than have a lot of small ones. I ask students to get to know their leaves so well that if we put them all in a big pile they would have no trouble finding their leaf. Great tie-in with Larry Lowry's leaf sort.