For stay at home working moms, how do you keep your toddlers busy while you work and no one is available to look after them?

My college friend who is working at home always make it a point to do her online job in certain areas in their home where she could see her toddler all the time. Oftentimes, whenever I visit her, her toddler is inside his crib (his crib is still big enough to be his play area) so that she can monitor him everytime. She knows what types of toys are in the crib so she is assured non of those can cause choking. She also plays music that her toddler loves listening to.