The old Pikachu pedometer for the old Gold / Silver used a watch battery. You could unscrew the back and replace it. The watch battery worked for me for a little over two years, I believe.
That was a great little device. You could get a lot of decent items using the mystery gift function in the old Gold / Silver - rare candies, evolution stones, etc. The pedometer was great if you didn't have many other people around to do that with.
I think that the N64 games also served the same function by giving you a mystery gift every day as well.
Anyways...I thought this old function was fun. Glad they're bringing it back.