Want to know the best route for the most bucks?
Get an ACE PT cert. Before anyone laughs and says ACE is not all that good, the point is that you don't care if it is or not.
Once you have the ACE, look into classes in your area for becoming a Medical Exercise Specialist. They teach the class here. It is 10 weeks, 2 afternoons per week.
Once you finish the class, you get a certified (easy - no test) and then you are able to work on referals from physical therapists and doctors. The best thing about it is that you can bill insurance companies for this time. Anywhere from $90-$125 per half hour. No need to even go to a gym. You go to the clients house or they come to yours. All you need is a portable heat/ice pack ($40), small swiss ball ($25), theraband "cable" things ($40) and those stress relief balls that you squeeze. ($5).
You can even work directly in a physical therapist office if you don't want to travel to clients home (but you lose the $15 travel fee) or if you don't want clients coming to you. However, the PT will take about 20% of your fee in exchange for allowing you to use the office. The good part here is - you have an official office environment to see patients, you have all supplies for you there, no driving in traffic, insurance is billed automatically by the office personnel, plus the PT's will give you referals from their own patients so you have a source of patients to work with. The only downside is you have to give them 20% ($20-$35) per patient you see. Which is not a bad trade off.