Hi Rhi, I'm in a hot area too and pretty much do what you're already doing. I get up early and do my walks or if I'm later I will walk for less time and less exertion.
I'll do aerobic DVDs under the aircon in the arvo or do an exercise show on PAYTV or use the elliptical trainer.
Is there a gym you could use nearby? I'd love to be able to do that but there aren't any around me.
Get a treadmill or something similar?
Swimming is great exercise - I'd be doing that as much as possible in summer!