All these rewards will remain on your account even after the ESO Plus Trial event expires.
Start your ESO trial from the ESO crown store.
Credit to a guy on reddit-
Here we go the complete list, hoping that could be useful for someone.
"Dark Brotherhood Initiate" Costume and "Assassin" Personality -> Complete the first Mission of the Dark Brotherhood DLC (The "Assassin" Personality only require to enter in the Dark Brootherhood Zone, but sometimes it is bugged and you need to complete the first Quest).
Skill Point and Skyshards -> That's sound obvious but it's good to remind it, you can gain Skill Point from Quests of DLCs like "Thieves Guild" and "Dark Brotherhood" or gain them trying to find the SkyShards in the DLCs Locations (YouTube or Google is full of Guides for finding them).
"Echelette" Pet -> Enter in the City of Orsinium (Orsinium DLC City).
"Jackal" Pet -> Enter in Hew's Bane (Thieves Guild City) (Sometimes it is bugged and you need to do it during the Quest that you obtain from Quen (You can find her in any Outlaw Refuge)).
Four unique Recipe for Scaling Food -> Orzorga Quest Line in Orsinium DLC.
"Heroic" Personality -> Enter any Dungeon of Shadow of the Hist DLC (You can go there with a simple Teleport and you simply need to enter, nothing else is needed).
"Executioner" Title and "Cadaverous Assassin" Polymorph -> Complete the "Litany of Blood" Achievement of the Dark Brotherhood DLC (Kill 15 precise Target in the World, YouTube is full of Guides about it).
"Master Thief" Title and "Coin of Illusory Riches" Memento -> Complete the "A Cutpurse above" Achievement of the Thieves Guild DLC (Steal 16 precise Item in the World, YouTube is full of Guides about it).
"Thieves Guild" Costume -> Complete the first Quest of the Thieves Guild DLC.
"Black Hand Robe" Costume and "Silencer" Title -> Complete the Main Quest Line of the Dark Brotherhood (It's a bit tedious but can be done in one day, the last Main Quest will be when the Skill Line is at lvl 8, doing that you can even obtain the "Shadow Rider" Passive from the "Dark Brotherhood Skill Line" that could be useful).
Also remember two things while ESO Plus is active:
You can dye your Costumes, so don't forget to dye all your Costumes how you like the most before the ESO Plus will end... The dye will stay even if ESO Plus is expired.
All the Crafting Materials will go in the "Craft Bag" (Even the ones that you already have in your Inventory/Bank), so try to obtain as much as Crafting Materials as possibile, they will stay in the "Craft Bag" even after ESO Plus is expired and you will save some space in your Inventory/Bank (After ESO Plus expire, the "Craft Bag" will remain but you can only withdraw the Materials, you cannot put anything in it).