Watermelon!!! That's what this could be! It's starting to vine so I know it isn't any of the varieties of zucchini or yellow squash I grew in previous years. I'm kind of excited to see what it grows up to be. Haha!
We used to spit our seeds under Grandpa's picnic table...and then he would have to deal with what came up! He was super old when I was very young so it was probably a pain to him to have to pull them up from under there now that I think of it.