First of all lets start with a small list of items you can buy during this event, these are bought from a vendor named Chapman, standing at all major city GYs (like most (if not all) other NPCs connected with this event does):
All items requires Day of the Dead, except Bouquet of Orange Marigolds (and the Recipe if you don't count the requirement on the crafted item)
Candy Skull - 25 copper
Use: Restores 61 health over 18 sec. Must remain seated while eating.
Orange Marigold - 10 copper
Use: Detect lesser invisibility for 30 sec. Only works during Day of the Dead. (30 Sec Cooldown)
"Said to attract spirits and help one communicate with the departed on the Day of the Dead."
Spirit Candle - 30 copper
Requires Day of the Dead
Use: Places a spirit candle that increases the spirits of those nearby by 4. (5 Min Cooldown)
Recipe: Bread of the Dead - 20 silver
BoP
Requires Cooking(1)
Use: Restores 2% of your health per second for 24 sec. Must remain seated while eating. If you spend at least 10 seconds eating you will become well fed and gain Stamina and Spirit for 15 min. (Stam/Spi increased by 25% of your level)
(To craft this a "Ghostly Cooking Fire" is needed, which are also found at all major city GYs)
Whimsical Skull Mask - 5 copper
BoP
Head
Bouquet of Orange Marigolds - 1 gold
BoP
Held In Off-hand
Use: Detect lesser invisibility for 30 sec. Only works during Day of the Dead. (30 Sec Cooldown)
"Said to attract spirits and help one communicate with the departed on the Day of the Dead."
Next the achievement:
This is easy as you just have to /dance with the NPC Catrina, which stands at all major city GYs
Coords:
Thunderbluff - 41,20
Orgrimmar - (About) 47,17
Undercity - 68, 11 Right when you enter from Tirisfal Glades
Dalaran - 35, 40
Ironforge, Shattrath, The Exodar and Stormwind all have these as well, however I don't have coords for the graveyards of these cities. Although I do believe Dalaran GY is around 35,40.
Finally the pet:
The pet was a bit trickier for me to attain. As wowhead doesn't have any specific information on this yet (didn't read comments very carefully)
However, there's a special ("named" as some call them) npc at each GY which can only be seen using "Bouguet of Orange Marigolds" or "Orange Marigolds", this NPC is called "Cheerful <RACE> Spirit" and this is the one which will give you the quest for the pet.
However not all spirits has this quest for you, as far as I can tell only the spirit of your home town (ie: Thunder Bluff for Tauren, Orgrimmar for Orc/Troll etc.)
This simple quest requires a Bread of the Dead (cooking recipe mentioned earlier) and gives you a non-combat pet:
Macabre Marionette
BoP
Requires Day of the Dead
Use: Right Click to summon and dismiss your Macabre Marionette
This pet isn't learned like most other pets its used from your inventory, and look like this:
That's all I've found so far.