My checklist of what to put in my workout bag is pretty standard and predictable: water, towel, workout shoes, sweatband, post-workout snack..... and my brain.
By "bring your brain", I mean that we have to drop off all the baggage we are carrying around in our heads, so that we can focus on exactly what we are at the gym for. Lose the worries over the project that's overdue, the babysitter that's just quit on you, and what will you make for dinner. All that stuff has no business taking up brain space and energy while you're at the gym.
When we clear our minds of all that stuff, we can focus sharply on our form (are we doing that exercise correctly and safely?), and our end goals (what do I want to look/feel like in 4/8/12 weeks?). Worrying about other stuff during your workout won't solve those problems, but it will have a negative impact on your workout and your body.
Sometimes we feel guilty that we aren't paying attention to the baggage during our workouts - have you ever thought you just don't have time to work out, because there are so many things on your plate? Avoid that rabbit hole and remember that you have a responsibility to the body you walk around in. A lack of brain focus will lead to a half-baked job of exercise, and won't get the results we want.