I always have a structure. I don't "wing it".
First establish my goal for the next phase (mostly size, strength, fat loss, power or a combination of strength & size).
From there I decide what type of training I will be doing (pump/fatigue based bodybuilding, strength work, hybrid, etc.).
I select the training split I will be using
So from there I know what (which muscle(s) or movement pattern(s) I will train on any given day) as well as the general style of training. When training for strength I also know the one main lift per day I'll be using.
The actual work being done (exercise selection, volume and special techniques if there are any) are selected on a daily basis based on mental state and gym performance. It's not random stuff or things I feel like doing; there is a reason for everything.