Treats are very much encouraged. They can be posted any time from now until creator reveals (at which point I close the collection) -- just go to the collection and click "Post to Collection" in the top right corner, then fill in the recipient's name on the relevant part of the usual AO3 posting form.

There are no specific minimum requirements for treats but in the guidelines I settled on "[they] should still be of a quality that you are comfortable giving as a gift" as a form of words.