Mixing isopropyl alcohol and propylene glycol
The two chemicals can be mixed without any reaction between them. However, you should add enough water to avoid fire hazards after the winter. Take a look at this table to find out, what mixture should suffice.

You should never pour solvents onto your soil or down the drain. Contact local authorities to find out, how you can safely dispose of that mixture. You can, of course, reuse the mixture and store it in some plastic cans if you need to empty the basketball hoop.