Muscle Gain?

Yes, lifting weights will make her buff without burning fat. Stay away from something like the treadmill.
I don't really know what the protein shakes do. To my knowledge they only give a moodlet but I could be wrong.

Okay, thanks! I will have her lift some weights. Because up until this point she's been doing the treadmill, yoga, and simba on the television.

Yeah, yoga will burn fat as well as increase muscle. The TV workouts both just burn fat. Your best options are lifting weights, basketball, and the punching bag.

Thanks again!

Be cautious of lifting weights if you don't want your sim to be over muscled. I had this active sim that was always wanting to workout. Eventually she got too thin and I've decided to make her lift weights instead. Now she's an over over muscled girl.

when my sim tried to only do cardio, the fat level mostly stays in the specific threshold mostly not below 40% of the slider, but if I added lifting to his routine, the fat can go below 40 up to 0, but be careful that body type can change if using lifting weight while the muscle slider is at 100%, my sims body all of a sudden changed from mesomorph to endomorph (aka bodybuilder body).

Punching bag and yoga combine cardio and strength training. They cap out at 60% muscle, I believe, so your sim won't get too bulky.

Not sure about the new machine that came with StrangerVille.

How about reading fitness books? Progress will be slower than actively practising the skill, but reading should leave both muscle and bodyfat as is.

If you want her to gain weight try the high calorie meals. My sim ate 5 in a row for 3 days and was near peak "fatness" of the fitness scale.
(Some calorie counted recipes listed here) https://forums.thesims.com/en_us/discussion/876976/calories-in-foods

Yoga is also an option I believe they patched so you didn't become a bodybuilder from stretching

And if you're okay with mods, mcc let's you "freeze physique" so they can work out without gaining muscle or losing weight.

Thanks guys! I'm going to take all of this and try and come up with the best routine for Bambi!