The best way I found is to use NodePaths. You can set these as export variables and drag and drop them in the editor. It will give you a relative path BUT importantly, it automatically updates if you move the tree around and you don't need to hard code anything.
export var other_node_path : NodePath
func _ready():
var other_node = get_node(other_node_path)
Then in the editor inspector you can click the icon and choose the node.