How about yoga? It's more of a workout than you would think (in some ways it can be compared to lifting weights, but your body is the weight.) People who do yoga regularly tend to be thin or slender unless they do other types of exercise that build visible muscle.

If you do yoga regularly, you will get strong all over, and your balance and posture will be great. If you decide that you want to start lifting weights later, you'll be strong enough for it.

I'm not going to give any advice about eating, since I'm at a place right now where I'm not yet able to think about food much ahead of time and make good plans. I tend to not think about food until I'm hungry.