There are two types: One with a Big Mac, fries, and a drink, and one with a 10-piece McNugget. The toys are weird though, and maybe not something you'd actually want.
They partnered with some fashion brand called Cactus Plant Flea Market. You get one of four figurines of the Hamburglar, Grimace, Birdie, and a weird new one named "Cactus Buddy". They're like old-school McDonald's toys, but with four eyes. (???) (Here's a photo.)
They're marketing them as Happy Meals for adults, but they're technically called "Cactus Plant Flea Market Meal Boxes". (Lame.) They'll hit participating locations on Monday and run through October.
By the way, that might not be the only nostalgic trick up their sleeve next month: There's also a rumor they're bringing back those Halloween Happy Meal pails that look like jack-o-lanterns. No official word yet though.