Fortnite XP Daily Summary

Here are more details on each of the major sources of XP:

Idling in Creative Mode no longer earns XP every fifteen minutes. You do however earn XP for accolades in Creative games, mentioned below.

Quests are the main way players will earn free in-game cosmetics and XP.
The following are only available in the Team Rumble and Battle Royale game modes (Solo, Duos, Trios, Squads):

• Daily Quests
  Players should see three Daily Quests available to them. Once they are completed, they should refill up to three available for your next round.
  Squadmates can complete (and also earn XP from) the active Daily quests of their teammates.
  Each Daily Quest is currently worth 1000 XP.
  There is also Daily Bonus Goal, which is awarded for the first three daily quests completed each day. They are worth 3x 15,000 XP.
  Daily Bonus Goals reset at 9am EST each day.
• Season Quests
  So far it looks like Season Quests are released on Thursdays. They are typically available until the end of the season.
  Each Season Quest is worth 20,000 XP.
  There are Season Bonus Goals which involve completing a certain number of Season Quests. Once they are all completed you advance to the next Tier of Goals.
  Tier 1 Season Bonus Goals are worth 3x 40,000 XP each.
• Milestone Quests
  Milestone Quests last for the the entire season, and reward you for completing in-game activities.
  Each Milestone Quest is worth 7,000 XP.
  There are Milestone Bonus Goals which involve completing a certain number of Milestone Quests. Once they are all completed you advance to the next Tier of Goals.
  Tier 1 Milestone Bonus Goals are worth 2x 15,000 XP each.
• Other Character and Story Quests
  Other thematic quests can appear weekly, and offer either XP or cosmetrics rewards. They may have an availability period before expiring, or more commonly last until the end of the season.
  XP rewards for these quests are typically in the 20-30k XP range.

Opening chests, ammo boxes, freezers, safes, llamas, drops award XP.
Collecting foraged items awards XP.
Eliminating players and other in-game activities will all assign XP.

Enjoy the game and the XP will follow :)

A first-person version of Among Us, this game mode consistently awards roughly 5,000-10,000 XP per game, depending how many tasks you complete and whether you win. If you do join this game mode with aim of earning XP rather than actually playing, be courteous to other players and make sure you at least Skip your votes promptly so as not to ruin the game.

Creative Mode maps can feature "accolades", in-game events or achievements which reward players with XP.
This is lower in the list because the XP awarded varies wildly. You can find videos online advertising "creative xp glitches", but the current reality is that they will generally earn meager amounts of XP for the effort, and can be very inconsistent. The game adjusts XP awarded dynamically, based on how fast you're earning, to discourage abusing the mechanic.
Your time is better spent simply playing the game and earning Creative XP along the way, rather than specifically targeting this approach to XP grinding.

The Save The World game mode features lower in the list, because it requires players to have bought it separately.
That being said, you can earn XP in Save The World mode which is counted toward Battle Royale and the Battle Pass. There is a daily limit in the range of 600k XP, so this is worth looking into.
It is possible to achieve AFK farming here if you are sufficiently far along ingame, using the Endurance mode.

This doesn't award extra XP so much as it catches you up on a little bit of the XP you missed by not playing the day before.

If you didn't complete all of your Daily Punchcard from the day before you may notice your XP bar glowing gold and labelled SUPERCHARGED XP. The idea is that supercharged status grants a 5x multiplier on any XP earned, up until you earn what used to be 30,000 XP. It's not clear what the current Supercharged XP limit is but it is likely below what could be earned doing the Daily Quests from the day before.

It's a way for you to skip a day or more of Eat-Sleep-Fortnite (seriously, go outside) and still keep up a bit of your leveling pace.

This is no longer a thing in Battle Royale.