What to do with $20k cash for 9 months out of the year when it is not needed for a seasonal small business?

Have a profitable seasonal side business that needs about $20k each season for start up costs. After we take our profits we put that $20k back for next year. So for about 9 months out of the year that money is just sitting idle in the bank account.

What can we do with it to make that idle money work for us without too much hassle?