IS 250 2008 (with Nav screen): Is there a way to change the climate control to fan only, or to override the thermostat to run AC even when it is below 65 degrees outside?

For example, it is very warm in my car, but 64 degrees outside, and the automatic setting wants to run heat only.

 

Yes, there is a way.
Using Toyota techstream you can change the personalized settings for climate control.
There's several other things you can change too, but you need Techstream.
I now have manual climate control on my Is350

Here's my recent thread on it:
https://www.clublexus.com/forums/is-...-question.html

 

 

Because every time you turn the car off and re-start, it reverts back to the original settings (very annoying).
And ultimately after constantly pressing that stupid little button every time I start the car, I was sure something electrical in the HVAC system would end up breaking over time. This is the same reason why I disabled the auto steering wheel tilt.

In my case it was summer and I just wanted to open the sunroof and MAYBE run the fan. It would always start the a/c as well as air recirc mode.

 

You can turn off the AC and leave the fan running, check your manual.

Maybe your car is not working right. The AC puts out hot or cold air depends on the cabin temperature as well. In my car, when it's hot inside, the air coming in is cool. Today, my car had been sitting out in the parking lot, when I was leaving work, outside temp is about 60 and inside temp feels like 76. My AC is set to Auto at 72 and the air coming out of the AC is cool/cold.

 

Sounds like your A/C isn't working right. Given the conditions you describe, if you push the "AUTO" button and set the temperature setting to a setting colder than the inside of your car, the A/C should turn on and cool the cabin. The temperature when the compressor cuts out due to low outdoor ambient temperature is around 35 degrees F, not 65.