Assuming indoor conditions (without sun and wind), at what temperature ranges (C and F) can infants (6-12 months) be left to play without clothes or in just a diaper?
Bonus points for outdoors in the shadows with light wind.
My daughters loved to be in the nude when they were babies. They giggled and wiggled like crazy and it was delightful to look at them.
When it comes to temperature, follow your instinct. Put your baby always on a blanket, because objects on the grass can vary from flowers to dog poop or broken glass. I would say, choose a really warm day, one of these days when you would be sweating on a t-shirt. Always choose a place in the shade and never leave the baby unattended. Touch her skin all the time to feel the temperature of her body.
The skin area-to-weight ratio is highest for newborns and decreseas as children grow. They lose heat faster than we do, but they also absorb external heat faster. Therefore, when the temperature's relatively high ~25C but the sun is shining brightly it may be OK to leave the baby to play naked. However, the same ~25C in shade may be much too cold for an infant.
Indoors, I'd say minimum between 25 and 30 degrees. It also depends on humidity - the lower it is, the faster the baby loses heat.
If you are certain the baby'll be cold, don't let it be naked. If you're uncertain, but the temperature's decent, let the baby play for a minute and check if it's cold. Do it again after another 5 minutes. And after 15. And so on.
There's little harm in leaving the baby for a while even if it turns out it's too cold. Arguably, it may make them ill with cold - but children sometimes get cold regardless of our attempts to "protect" them.
So just stick to your instincts and remember to check on your LO from time to time.
If the baby is old enough to be toddling around and there is a warm breeze, mid 70's Fahrenheit. Infant, upper 70's, shade, warm breeze. I would always keep a receiving or other light blanket around. If the baby has goose bumps, put a t-shirt or onesie on the child unless they appear to be just a random thing.