I have three little boys, so believe me when I say that I know what a mess looks like. Sadly, I've seen gyms that look like a hurricane of toddlers have devastated the place. Weights everywhere. Machines in disarray. Trash in the cup holders and on the ground. Y'all know your mamas taught you better than that. You should leave the place as you found it, perhaps even better if you see an opportunity for improvement.
Treat equipment at least as well as you would if it were your personal property. That stuff costs a lot of money, and breaking or otherwise abusing it will only cause those expenditures to be worked into your monthly membership fee. Don't drop weights on the floor. Unless you're an Olympic-level lifter, you can set them down nicely. If anything, a slow drop will improve your triceps, and that's always a plus. Always rerack the weights when you're done. And don't leave the weight bar with weights still on it. Not everyone has the strength to remove the weight that you – superstud you – were able to bench-press.
Finally, if you're tired out from your leg press set, resist the urge to rest while sitting at the machine – or worse, to make a quick phone call on said apparatus. Unless the gym is empty, stand up and give someone else a chance to work in.