If you’re a returning player, check Ikeda_kouji's post out or watch Laranity's video to see what’s been added in your absence, and to decide on what content you'd like to purchase. Also check the Story Section to see if you're up to date. As a reminder, Living World Season Complete Packs will save you money as opposed to buying individual episodes (In Gem Store, under Accounts -> Living World)
Birthday Gifts - If you’re a returning player, your characters receive a birthday gift in the mail each year after they were created. Here’s a breakdown of gifts:
Birthday boosters - save for times when you'll need it (PvP/WvW reward track bonus, magic find for events, etc)
Any minis can be right-clicked and added to wardrobe. Same thing with mail carrier/victory rock, backpacks, luminous weapons (you can use these in their item forms to have a free-to-apply skin once so you don't use transmutation stones but obviously they'll take up space)
Bag of spirit shards - open 'em
Teleport to friends - save them. You can use them to port a character into expansion/Living World Season areas without spoiling story.
Birthday cards - use 'em right away. Just free karma.
Black lion statuettes - can be traded for a bunch of things that are on gem store. Save them up in your bank.
Guaranteed wardrobe unlock - use 'em right away and pray you get something cool haha
Amulet of Celebration (8th Birthday) - Stat-selectable Ascended Amulet. Make sure you right-click and customize to select stats and no, you cannot change the stats later on. Choose carefully.
Celebratory Birthday Enrichment (8th Birthday) - Goes into any ascended amulet with an enrichment slot. Can be freely removed. Has a fireworks effect when you draw/stow your weapon or enter combat.
MOST IMPORTANTLY - Look up the dye packs to preview what colors you want OR you can do what I do since I suck at fashion wars and pick the most expensive dyes. Check wiki for prices (ex. https://wiki.guildwars2.com/wiki/Celebratory_Dye_Pack). Scroll down to the bottom to see the other birthday dye packs (3rd through 8th birthday)