You could always do what some people we knew did after the youngest of their 4 left for college. They adopted 5 - yes 5 - little ones. I think they had 5 under 5.
For me borrowing 5 under 5 for a day would have probably cured any empty nest syndrome! But to each their own.
I hit the empty nest syndrome early because her sophomore year my younger one decided out of the blue to apply for the residential State math/science school. We were really surprised as she was a good swimmer and loved band and the math/sci school had neither of those. So it was more of a sudden shock and not the gradual lead up. I was pretty depressed and shed many tears most of her sophomore year at the though of her leaving at 16. For us College is actually easier as she is much happier (the math/sci school was not a happy experience, very stressful) and we can easily to communicate with her compared to the math school where they had little access to phones.