Backstory: I switched gym's recently and the amount of people that don't use lockers in the gym is impressive from a trust standpoint but frustrating when trying to find an available locker.
The Idea: A sensor inside gym lockers that toggles a green/red light on the front of the locker to indicate it's open.
Deep Dive: You would build a sensor on the inside of the locker that would determine if any clothing items are in the locker. The sensor would toggle a green/red light on the outside of the locker, letting people know if the locker is available.
Validation: Next time you're at the gym, look at how many people go down the line of lockers to find a free one.
Monetization: You could try to sell this to gyms but they probably wouldn't see the value in spending extra money for something that doesn't solve a big pain point for them (customers have already paid). You could offer this for free to the gyms in exchange for advertising space. This would be either on the lockers or in the locker rooms themselves.
Steps to Launch: By offering this for free to the gyms, I would try to find an advertiser that's willing to cover the cost of production and then find a gym that's willing to test out your product.