It's the 20th anniversary of the release of Diablo, and the various Blizzard games are celebrating. These are the events I've tried.
World of Warcraft
In WoW, there are Treasure Goblins which spawn in the Broken Isles, in the Dalaran Sewers, or at the end of instances. Kill them fast enough, and you can take a portal to the Cow Level, which has a lot of diabolical tauren, cows, and the Cow King. Killing the Cow King gives you a toy.
I recommend going for a Treasure Goblin in the Dalaran Sewers. You can pick up a guard from the guard captain for 5g, and that will keep you out of PvP (for 5 minutes or so). There are usually lots of players in the sewers, making it easier to kill the Goblin.
You can only loot the Cow King once, no matter how many times you kill him. However, you can loot multiple Treasure Goblins. Apparently the Cow King is a mess on PvP servers, but that's what you get for rolling PvP.
All in all, a short, fun event with some Diablo-themed loot.
Diablo 3 has a small area and dungeon which mimics the original Diablo. There's a filter making all the graphics look old-school and pixelated, though all the mechanics are still Diablo 3. There's 16 levels in total, and four old bosses.
I recommend that you start with a new level 1 character in an Adventure game on Normal. You can portal right near the entrance to the new area and enter it without killing anything. Completing the dungeon on normal with that character gives you an achievement and pet. Plus it gives you the enjoyment of low level gearing and gaining levels and abilities.
One thing I wasn't sure about was if you were allowed to go back to town before finishing and still get the achievement. I ended up just dropping all the blue and yellow items that I didn't equip.
I did it with a new Crusader, and it was pretty easy. If you're thorough, you can get almost all the seasonal achievements in that one run. The only one that takes multiple runs is the achievement to kill all the unique mini-bosses.
This was a lot of fun, and the old-school graphics did a good job at invoking nostalgia. And at the same time subtly pointing out that normal D3 is a pretty good looking game.