Natural light only, BUT it can't be direct light.  Diffuse it, bounce it, color it, etc.  No restrictions on camera, lenses, or subject, just that you can only use natural light.  No artificial lights of any kind.  You can modify the natural light by bouncing or diffusing it so long as you explain the 'how and why' in your post.  This one is going to be a bit of a challenge for everyone, but it'll help with next month's project too.

Inexpensive DIY light modifiers: white/black foamcore ($1-2 a sheet) or matte posterboard, sheer fabrics, colored saran wrap, aluminum foil, waxed paper, frosted shower curtains, etc.  If you have a bright enough light source, you could probably get away with even using tracing paper for diffusion.  Hell, a $3 Ikea Shottis blind would work decently with bright light and it folds up.

Honestly, if you're serious about learning to play with light, I'd consider getting a $13 5 in 1 reflector off Amazon as they're pretty darn useful.  You can get a larger with handles one for $20 as well.
To be clear: buying one isn't required for this challenge.  Just listing two of many options in case you don't want to DIY it.
https://www.amazon.com/Etekcity-Collapsible-Multi-Disc-Photography-Reflector/dp/B00DIHSZCC
https://www.amazon.com/Selens-Portable-Reflector-Collapsible-Photography/dp/B00UR9AS9W
https://www.ikea.com/us/en/p/schottis-pleated-shade-white-90431378/

That said, if this seems to complicated I can change it up.

I think it's a great challenge, you might just have to give people a little bit to work with it to come up with something compelling.  Amazon Prime right now isn't even terribly prime, and we can't go to IKEA.  Today I'll be doing outdoor activities to comply with my governor's directive so maybe I'll find something there.

If you want a simple diffuser that some might have around the house. A sheer curtain makes for a one.

This could be fun!

Good challenge, NP!!

Reflecto-Dome by iroc409, on Flickr

Not quite what you are asking for so I'll try and make another entry, but it kind of fits.  Here's how I did it:  I used the sun to light the bright side of the dome, then used the mountains and the sky to illuminate the shadow side to provide a coloring of the dome.  Just hold those mountains right up and allow the sky to diffuse the light.  

D500 with 300MM F4 PF and 1.4x TC III.

Nice!

Sunlight on a sphere has always kind of intrigued me.

Nicely done.

Indirect, natural light, product photography.




Edit:  Forgot to explain how I did this.  I use white foam core and poster board for the base and background, and lay them out on my living room floor.  Then, I simply open the Venetian blinds so there is plenty of light coming in the room, but not so much as to get  direct light.  The walls in my living room are an off white color, so basically the entire room becomes a big light box.  The camera is on a tripod, and stopped down for adequate DOF, and the exposure times are around 1 second or so.  The hardest part was editing out all of the scuff marks on my dirty sheet of foam core.