Finding something for your Thai girl to do
I like the idea of sponsorship agreements, live in girlfriends and hang-a-rounds and I'm OK if the girls are not currently employed in fact it's better if they don't so you can get the most out of your agreement. Not being employed allows a sponsor more flexibility to be around when you need or want them too. But these girls can't entertain you all the time and when you're working or just need some time alone if the girl you have doesn't have something to do it's going to get old quickly. This is useful if the girl doesn't currently go to school as well. You need to find something for her to do.
Now I should probably be going to the gym myself but this isn't about me getting fit in Thailand it's about getting the girls as fit as possible especially if you're going to get into a long term agreement/relationship with a Thai girl I shouldn't have to explain the obvious benefits of doing this.
To get a Thai girl to go to the gym and actually do some exercise is going to be as difficult as it would be for you to try to convince me to go as well. Isn't going to happen. You could try the bargaining or payments but you're still going to get the complaints and you're going to have to keep nagging her to go, not ideal.
So what are the alternatives?
Thai girls and Thai's in general need something to be fun no matter how menial the task if they aren't having fun they aren't doing it. My suggestion is to get your girl to dance.
Dance lessons for Thai girls are a great idea. This is not only going to help her get fit she's going to be surrounded by a bunch of kids and is unlikely to stray from your path. Dancing will also give her more confidence especially if she's young or extremely shy. Also by learning how to dance she can also prepare herself for the eventuality of her working in the future when you decide it's over.
I don't see any benefits to getting a Thai girl to go to English lessons except if you ever want to leave the country (with her) or if you run a business together here that requires her to use English. English classes are a good way to to get her out of the house again but if she isn't first trying to practice English at home it's not worth it. Besides you should be learning Thai!
The alternative for your Thai girl not having a job and having nothing to do can cause your Thai girl to become:
To avoid this outcome get your sponsor active by signing her up to dance classes.
I recommend the following Dance Schools:
Chiang Mai: Hug Academy
Pattaya: Steps & Rhythm
Bangkok: Studio Zoom
Phuket: Phuket Danz Steps