Try Gun Game for Fun Sessions
When I want to play Call of Duty: Black Ops 3 to chill out and still earn some Cryptokeys I play Gun Game.
This is in the Bonus area and it switches you to a new weapon with each kill. Not only are you earning XP with multiple weapons, but you get a lot of variety and you're playing against people who are matched up with similar weapons, not a player with four perks and a spec'd out gun that puts yours to shame.
Gun Game isn't for everyone, but it's one that I find very easy to jump into even when I only have time for one or two games.
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