Really don't even need a device. I practice balancing on one leg with my eyes closed while brushing my teeth every morning (I've progressively made it harder over time). I mostly do this to keep up my balance skills for mountain biking and it's helped quite a bit, especially during winter when I ride less.

on the opposite end of simple-or-cheap, e-foiling Does train your balance. esp. in windy "cabbage-salad" waves with medusas and rubbish floating around :/

Does it though? Balancing doesn't seem like an active cognitive process, like I don't have to think about not falling over to stay upright on a bicycle. All the little shifts of body weight needed to not tip over seem to be done automatically.