card sleeves for trading cards Standard Soft allowing them to easily and securely fit into any binder for safe keeping. Performore 9-pocket Trading Card Binder

This is useful in a huge variety of situations, and while its not repeatable like others on the list, it is cheaper than some of the other choices

Weird Angel Flame, for example, grants a heroic ability, but it can also grant protection from (checks notes) even mana values

But again, this will really vary

as long as you had answers to the cards you let them keep on top of their library

It saw niche play in Standard at the time, but was way more relevant in EDH and still is today

This process of swapping out weaker cards for more powerful or synergistic alternatives is how you begin to personalize your deck and raise its power level