It would be a good idea to get a set of level 80 exotic equipment if you haven’t already. You can craft it (if you’re at or near level 400 in the relevant profession/s), buy it from the Trading Post with gold or buy it from Temples in Orr with karma or WvW with Badges of Honor or get it with dungeon tokens from the merchants in Lion’s Arch. Or a combination of all of the above if you don’t have enough of one currency.
Other than that what you do is up to you. There’s not really any new game modes or drastic changes at level 80, but the game becomes basically free-roaming. You can go anywhere in the game and do (or not do) anything you like without having to worry about if it’s the right level or going to help you level up.
Danielle Aurorel, Dear Dragon We Got Your Cookies [Nom], Desolation (EU).
“Life’s a journey, not a destination.”