What is this plant?
+5
Looks like it could be squash or pumpkin variety but its very viney rather than bushy like my squash plants.
Could it be a cucumber or a different vining cucurbit? Have you noticed any blooms?
You have a volunteer! Wait and watch for the blooms on it and you will know soon enough what is growing. How exciting. I love volunteers. I have some annual flowers that came up again this year in my garden and even that was exciting to me as a gardener. It does look like some kind of pumpkin, gourd, or melon if not cucumber. What an intriguing mystery!
I THINK IT IS A WEED
Looks like a baby grape vine!
I agree with most of your comments, let it grow and you will have some kind of squash melon or Chayote...
Chayote is an edible plant belonging to the gourd family Cucurbitaceae, along with melons, cucumbers and squash. Globally it is known by other names including chaya, chayote or pataste in Nicaragua, christophene in Trinidad and Tobago, christophine, cho-cho, sayote, pipinola, pear squash, vegetable pear, choko, zidra in Ecuador. Wikipedia
You can also bake them like a potato.. If you microwave them, stab it with a fork a few times, to release the heat... or the will explode.
Great with salt and butter like a baked potato!