You can start growing pumpkins now (starts are available from Leif and Nook's Cranny) and use them in pumpkin-based DIY projects. Costumes are available from the Able Sisters, and you can exchange Nook Miles for creepy face paint and contact lenses
Don't forget to stock up on candy for the Halloween celebrations -- Nook's Cranny will sell you one piece per day.
Come Oct. 31, your island buddies will dress up, gather in the central plaza and demand candy -- failing to give them those treats might result in you getting pranked (I kinda want to see that though). You'll also get a new visitor: Jack, the pumpkin-headed "czar of Halloween."
You'll find that Jack has already mailed you a painting of himself, which you can hang on your wall to remind you he's coming. Downloading the fall update also gets you an in-game Ring Fit Adventure controller decoration -- the Switch fitness game became incredibly rare in real life this year, but is back in stock now.