In addition to some story text, the only activity is to claim your "Grey Year Souvenirs" from the yellow box towards the bottom of the page.
Each day from today through June 21, an item prize will be made available to all players, and Premium players are eligible for an additional bonus item prize. Today's regular prize is The Grey Year Brochure, a brand new book item; the Premium prize is Desk Yooyuball, a re-released toy from the 2019 AC Staff Tournament prize shop. Additionally, a brand new "Dizzy Yooyu" avatar is awarded for claiming your prizes.
Unlike similar events of the past, you do not need to check in every single day; in fact, according to the official FAQ, all regular item prizes will be available until next year's Altador Cup and all Premium item prizes will be available until the end of June. However, the FAQ also says there will be a trophy awarded only to those who claim all prizes before the end of June.
Finally, the Neocash portion of the event, Altador from the Clouds, has also begun today and will be available through June 30th.
