There's a whole lot of optional content at 50 (since that had been level cap for a long time until the expansion came out). As Naria mentioned, you'll want to complete at least up through the end of the 2.0 storyline (culminating in the quest "The Ultimate Weapon") because that one unlocks all that optional level 50 content.

1st thing: complete your level 50 job quest
2nd thing: complete the MSQ up through The Ultimate Weapon

after that, you have quite a few viable choices:

Some people like to keep moving ahead through the next part of the Main Scenario right away. There's roughly another hundred or so MSQ at level 50 that were added in the 2.1 - 2.55 patches. If your goal is to get to Heavensward as soon as possible, this should be high on your priority list as these quests are a prerequisite to entering Ishgard and the Heavensward content.

There's also a variety of Side Story Quests you can undertake at this point if you think it's a good time for a bit of a break from the main scenario.

There's optional dungeons and raids as well. (Can't find a page that just lists these ones specifically, but you can look into all the stuff listed as level 50 in this link.)

Some players also choose this as a good point at which to go back and level up other classes/jobs they're interested in, particularly if they were focusing rather heavily on just one so far.

Since most players will want to do at least a bit of each of those, it's mostly just a matter of preference as to the order you do them in.