What do you do with CapEx money?

[Marty Gold]

I am searching for my first rental and know it's important to save CapEx money each month for big problems that eventually arise.

But what do people do with this money?  Stock market is likely too risky and bonds aren't liquid enough.   Do you just leave it in a savings account earning next to nothing for years or maybe decades until one day you need a new roof? 

Thanks