Best ways to level up quickly in Adventure Mode in Fallout 76!
Lunchboxes and XP Events
Keep your lunchboxes. Do not use them unless there is an XP event. Double or Triple.
Once it starts you need to stack 4 lunchboxes to get 100% XP boost on top of the event. it does not stack with Food.
You can buy those for 100 gold bullions a piece at faction vendors at Crator and Foundation. You can earn them as rewards for completing Seasons, you can buy them for Atoms in Atomic Store (which I don't suggest you do).
They are also AoE, that means if opens them in your proximity you will get the full boost of 1 box. So if you have friends playing you should all group up and open them together to have more uses for each other.
If you have no friends, then you should go to popular events like Earl or SBQ and people there often open lunchboxes for everyone at the start, sometimes closer to the middle of the event.
Then go off grinding high XP areas without losing much time, don't forget to equip "Inspirational" the XP group perk and join a group for faster leveling.
You can try using XP cranberry relish for food 20% exp gain bonus with herbivore but it is better to just do SBQ or similar to get lunchboxes off other players.
Events give a lot of XP, preferably the ones that have lots of enemies like Project Paradise or Uranium Fever. Stay in Casual Group for XP. You aren't after the loot. Be efficient, try server hopping.
Popular Locations with High Level Enemies
These are locations that are available to everyone at the same time, they cannot be private and will often be found empty, and you will have to server to repopulate them. The offer the highest amounts of the highest leveled enemies at once.
Some of them include:
Solo Story Quest Dungeons
If you are tired of popular high level spawn locations being empty because other people are grinding them then you can always fast travel to a private story dungeon like Vault-Tek university which you can enter solo even when you are grouped.
The result is that all enemies inside are alive. They will be slightly lower level but you will still come across levels 75 etc. This offers consistent spawns.
Often, these locations are unlock only after completing their designated quests. Those include Main Questlines, Ally Questlines, Side Questlines. Some of these locations as examples below (click to enlarge):
Daily Ops offer the constant respawning enemy flow, but at reduced XP rate and lower leveled enemies. However, this is the easiest to sustain as they will drop ammo and stimpacks. Some days they are extremely easy, whereas other days they can be annoying when you have to fight against freezing or only melee kills perk rolls.
They can be accessed from the Map's lower left corner by clicking on the Daily Ops sign.
Season Points Repeatable
This is great because not only will you be able to level up a lot, get more perk coins but you will also be able to get a lot of 10k/10k EXP gained for Repeatable Season Challenge to get 100 season points to level up your season. They really do stack up.
Enjoy the grind!