My favorite one is on the path between Fortree and Mauville. There's a bridge across a river about halfway through the map, and if you sail down the river you find a tree. The left (maybe right) side has one of those huge bases with the holes Plenty of room for slides, stages, and things that explode.
Does anyone know what happens if two people who have secret bases in the same places mix records?
You can visit the secret base of your friend.
But I don't if you can do anything special in his secret base.