I have a pool at my beach house overseas and had to add more information on my contract based on prior negative experience. I assume the process will be the same for gyms.
I would start writing an Exhibit to the Rental Agreement, "which is an integral part of this Agreement". And use this process:
1- Liabilities: Ensure you transfer the liability to the guess for the use, and always restrict (uder adult supervision) or prohibit children (at the gym), if there is a problem, it is their problem.
2- Identify what are your largest wear and tear items, I had to include that if they go in the pool without washing off the sand and damages the pool filter or clogs the system, they will be charged.
3- What are the thing the guest can abuse (such as leaving the jacuzzy pump on at all times, etc) and define what is acceptable and what will be charged to the guest
4- Have the guest accept the equipment in working condition within X hours of check in, or they will be responsible of any breakdowns.
I have my beach house in a country where electricity is extremely expensive, so one way I control abuse is by limiting the energy usage to X kWH, and the guest pays any excess at $0.50/kWH, so if they abuse any equipment, they will end up paying in electricity. This may not work in the US market.
We have never had any issues with regards to our pools, hot tubs or gym equipment.
We do tell our guests both verbally and in writing that if they use excessive lotion before going into the hot tub, the spa will turn mucky. They will then need to pay for an additional cleaning during the week.
We also tell our guests that they cannot touch the controls. If they do, and things break, they will be liable.
We have wonderful pool service companies handling our pools and spas on a weekly basis and find that rarely do we have them abused. Hence no addendum to our agreements.