I,ve grown melons,courgettes and last year butternut squash, it gets cold on this hill so the GH gives them protection

You can also grow carrots, peas, in fact most anything you would grow outside to get an earlier crop. Really it depends on the size of your polytunnel.

my polytunnel is 20ft X 12ft.

That is a pretty good size Steve so the veggie world is your oyster!