iserith
Magic Wordsmith
Sure you can. If a character has to parkour up a building, or something like that, you can set the DC before hand based on the material, how many outcroppings there are, hand holds, etc..., but, more to the point, how hard you want it to be. Even if it comes up on the fly, the DC can be set on the spot based on your idea of how difficult it is.
If the player then, as you said, does an action that can make it easier to succeed, he can be granted advantage, which gives +5 to passive skill checks. Conversely, if the player isn't specific enough or takes an action that will make the check harder, giving him disadvantage (-5 to passive skill checks) or unable to succeed at all.
Either way, its very easy to do this without deciding the result ahead of time, you simply set the DC. If your players have a character (or characters) that can automatically succeed at this based on their builds, then that rewards them in a non combat way for their class being good at certain skills, which is nice for positive reinforcement.
Perhaps we're talking past each other then because I'm advocating not setting a DC until the player has stated an approach to achieving a goal. "Parkour up the building" is an approach to the goal of getting up to the roof or balcony or whatever. You can set a DC for doing that task in your notes if you want, but it's potentially a waste of effort since you don't know if the player have his or her character engage in that approach. The player may decide to throw a rope with a grappling hook, take the time to hammer in pitons and climb up, or cast a levitate spell, for example.