Many moons ago, I offered to run an OSR campaign for D&D players who hadn’t played anything other than 4e. They took a long time to learn that they could try to do stuff that wasn’t specifically listed on their character sheets. They took a long time to learn the three most important words a DM can say to a player:
“You can try.”
“You can try” is your answer to just about everything a player wants to make her character do. “I want to leap the chasm.” “I want to shoot the ogre in the eye.” “I want to surf down the stairs on a shield.” Pretty much any time a player says “I want,” you can reply with “You can
In order to facilitate their “try,” you need to be able to do adjudicate off-the-cuff skill checks, assigning a reasonable DC to what they want to do. “I want to leap the chasm” is Athletics. “Shoot the ogre in the eye” is a basic attack roll, with a crit giving the desired result. “Surf the stairs” is Acrobatics.
Now, be forewarned: Once the players figure out they can try whatever they want to do, they’ll start chaining stuff together: “I want to leap on a discarded shield, surf it down the stairs, and shoot the orcs.” You’ll be tempted to simply say “No.” Resist this impulse. Let them try! Rather than “No,” say, “Okay, Legolas, sure. Let’s do this.”
At this point you’re probably shouting, “How, Bob? HOW?” Easy. Assign a skill or other check for every comma in the player’s stated action. “I want to leap on a discarded shield” – Athletics/Acrobatics – “surf the stairs” – again, Athletics/Acrobatics – “and shoot the orcs” – standard attack roll. Write it out if you have to, with your grammar governing where the d20 rolls happen.
Also, keep it within reason. Your players, even when they’re trying cool stuff, are still subject to action economy. Remember RAW for what a creature can do on their turn: Basic/minor/free, Move, Action. The stair-surfing stunt is no more than that! “Leap on the shield” – minor – “surf the stairs” – move – “shoot” – attack action. If the player wants to do more than that, you’ll have to have her split it up. Usually, “more than that” equals wanting to simply do too many things or move too far in one turn.
NB: I have been known to let players spend Inspiration to try something more complicated than Minor/Move/Action, but it has to be super rad. That’s a judgment call, and I make it clear that players are not to invoke that too often. Try it, but do so at your peril.