I agree with Rusty.
Since the author writes the way she would speak, she often depends on the context to be understood.
She could have written: "Lo ideal sería que restes tiempo de actividades como ver la televisión o usar el móvil y lo inviertas en ejercitar tu cerebro", and this "lo" would be immediately related to "tiempo".
Spanish is flexible like that, so most native speakers will know what she means, but her style could be neater.
