I would appreciate advice on how to summerize my spa. Here in AZ there's no reason to keep it running June, July, Aug when air temp is too hot to enjoy a spa. I drained the tub, but I'm wondering about the remaining water in the plumbing, just sitting there all summer with who knows what growing in it.

If the spa is fully drained there won't be any significant amount of water left in the plumbing and any drops that are left will evaporate in a couple of days.

There is an issue with new spas that are wet-tested in that the water that remains in the plumbing can often form biofilms, but in your case if you use chlorinated water when you are draining and the temperatures are hot, then you are at lower risk as Jason mentioned. New spas are often stored in cooler warehouses or showrooms so evaporation of water is slower. Worst case, you can just re-open your spa in the winter using Spa System Flush if for some reason biofilms formed in the piping from any residual water. When you open your spa you can just see if your daily chlorine demand with no bather load is the "normal" 25% 24-hour loss at hot spa temperatures.

I don't know how hot it get's over there, but last week it was over 35 degrees in my garden. I shut of the power of my spa, added cold water till it was 32 degrees and spent a wunderfull afternoon in it

However, on refilling I would also clean the pipes with a special cleaner.