How do you handle item interactions in your game? What does you consider to be an Action and not? As stated on the PHB, drawing a weapon is part of the Attack action, but it doesn't say anything about putting a weapon away and drawing another one.

For example

1 - Put away a shield + sword and picking up a bow in your back. Do you allow all that for free or it would be an action?

2 - Picking up a potion in your backback and then drinking it - 1 action or 2 actions? 

3 - Casting Darkness on a object. Then, can you cover/uncover this object as a free action, then do your actions or to cover/uncover it should be an action? (useless if so)

4 - Using your crossbow to shoot someone and then put it away, grabing your bard instrument and use as a bonus action? Allowed or not?

The thing to keep in mind is that, on each turn, a character can have one minor item interaction. This mostly includes simple activities that don't require precision... opening an unlocked door, picking up an object on a table, pulling something out of your pocket.

Doffing a Shield is overtly given rules that it takes 1 action... think of it under the assumption that a shield is secured to your arm with straps and not just something you hold onto by a single handle. So you would need to take a full action to doff your shield, and use a free item interaction to sheath your sword, so you would not be able to wield a Bow until your next turn, but on that turn you can draw the bow as part of the attack action.

Pulling the Potion out of your backpack is your item interaction (even if it doesn't fully make sense... how do you just reach back and pull something out of your backpack?), and drinking the potion is an action.

For the Darkness concept, Covering/Uncovering the object is your item interaction on subsequent turns. So you can't cover the object, make your attack while the darkness is supressed, then uncover it to gain the benefits of being in darkness, because that is two separate item interactions.

For the crossbow situation, you cannot stow your crossbow and then draw your instrument in the same turn. However, you dont' actually need to stow your crossbow in order to access your spellcasting focus. You can be holding your crossbow with both hands to fire it, then hold onto the crossbow with a single hand while your other hand uses your item interaction to grab your bard instrument. Even though it doesn't fully make sense, you don't actually need to play your instrument, you just need to have it in your hand to cast spells.

You can draw or stow/sheathe for free, but can't do both without using your action. If you have a two-handed weapon, shifting it to one hand so you can draw a different weapon is okay. But if you have a one-handed weapon and want to draw your bow and fire, you can't sheathe the weapon, draw your bow, and fire.

Many get around this limitation by simply dropping the first weapon on the ground, which doesn't use up your item interaction. But some DMs annoyed with this loophole might put you in situations where dropping a weapon may mean losing it forever (and I might be one of those DMs).