Log into the game on November 2—BlizzCon Day 1—and you'll be given a free golden Star Aligner card, and a golden Secret Plans card. Jump in on November 3, the second and last day of this year's BlizzCon, and you'll also get a free Boomsday Project card pack. The freebies are completely separate from BlizzCon itself, so you don't need to worry about springing for a virtual ticket or anything—just log into the game at the appropriate times, and the cards are yours.
BlizzCon 2018 will also include a Hearthstone developer panel, a "streamer showdown" featuring Trump (the streamer, not the other one), Alliestasza, Brian Kibler, Kripparrian, and developers Josh Durica and Keaton Gill, a look at past and future solo adventures, the Hearthstone Global Games finals, and livestreams that will run throughout both days.
That's on top of the other BlizzContent for games including Overwatch, Heroes of the Storm, World of Warcraft, StarCraft, and Diablo—although probably not too much Diablo. A full rundown of BlizzCon 2018 and links to purchase virtual tickets so you can watch all the livestreams (although as usual, the opening ceremonies will be free for all) can be had at blizzcon.com.