Not really much besides items. They added in the main events and items along with them, mario items, more seasonal items, sanrio villagers and their items, customizable standees and fans/umbrellas. This month we got new a may day maze and the art section included in the museum stamp rally.
Honestly the Mario and Sanrio items are pretty awesome and definitely worth a look. Pretty crazy that there are like.. 90(?) Sanrio items that were added. There are also some other seasonal items in the Nook Miles machine that were pretty interesting. I'm guessing you played when they added the extra item storage in the house to 2400, but if you haven't seen that yet, that's honestly one of my favorite additions. Besides that, the extra design QR slots were probably their best addition.