Has seen quite a bit in their time as a seasoned adventurer and thus have collected quite a bit of knowledge that a first timer wouldn't know. You've even picked up some magic that a classic bard wouldn't know, mainly the nature magic of the druids... along with a few spells that just seem like they would be helpful to know. (Instead of using CHA to cast, like a bard usually would. An adventurer uses their vast knowledge making them a INT caster)

Bonus Proficiencies

As an adventurer you have a lot of journals filled with information on a whole lot of things you've experienced, maybe if you sit down and reread a few of them you can gain some deeper knowledge.

Quick Witted

Also at 3rd level, you can use a bardic inspiration to have a sudden proficiency with a weapon or give a party member a weapon proficiency until your/their next long rest.

The inspiration allows you to use your wit and think back to a book or journal you have about weaponry and quickly think of how to use the weapon properly quickly giving you or one of your party members the upper hand with the weapon, thus giving proficiency with the weapon until a long rest

Additional Magical Secrets

You decide to go and look at those scrolls and journals you were writing when you were studying different magic users. From Wizards and Sorcerers, and even a few Warlocks. Certainly if you dig through these old things you can learn a spell or two.

Fearless Traveller

Starting at 14th level, you have decided to delve back into your basic notes and journals you were writing when you just started being an adventurer. These old journals and scrolls bring you back to the good ol' days, but they are actually filled with wonderful notes for surviving and getting through tough areas, as well as ways to patch yourself up in a bind.

(grants advantage on medicine, survival, and athletics checks)