Volunteers are always fun if you have the space. Most of the options you listed are completely different species with completely different seed leaves. If you take a picture of the mystery plant I am sure I could eliminate a lot of your options. Though I imagine you enjoy the surprise. The mystery plants are always hit or miss with me. I have had some that far out perform the ones I intentionally plant and others that take up space and are just boring (like an all white nest-egg type gourd).

Thanks. Its not very big yet and I'll get some clues in a few weeks.

As I said, I enjoy the mystery. And figuring it out will probably be a good project to do with my 9yo garden helper.

Waiting for it to grow up is part of the fun. :-D

I moved my mystery cucurbits today. Both have a watermelon/cantaloupeish look to the leaves.

If they live, it will be very interesting to see how they come out.

Watermelon ine and leaves are distinctively differen from the rest.
They(leaves) are much smaller, with pronounced lobes from early on and darker in color. Cantaloups/melons leaves are more round and smaller than squash's.Also squash grows much faster than cantaloups.
If you post a picture it would be easy to identify them.

I'm happy to wait for them to reveal themselves as they grow. :-)

I always enjoy the unknown factor of my mystery cucurbits -- sort of like how we never wanted to know the sex of our babies until they were born.