Nothing much eill grow nder a walnut tree. If I had more room and was younger I would plant black walnuts. Or try to make a hybrid black / english to get more of the black flavor but easier to crack. that may not be possible
My dad was enthusiastic about them long ago. I don't remember if I ever ate them or not. We also had black walnuts. I like them much more than the English walnuts, but it's a lot of work getting out the nut meat.
I think monkey puzzle trees have a narrow range they can grow in. Something similar to maritime climates, mild and not too hot, not too cold. They grow here. I've seen some beautiful ancient specimens. But mine died.
My dad grew a butternut tree. They have them at Raintree nursery. I don't know much a out them otherwise.
I have grafted apples and pears. I used root stock that grew originally as suckers from other grafted trees of the same species. i'm saving some maple volunteers to use as rootstock for nice established varieties of maples, but that is a project for later. Grafting season is over for this year. Budding can be done in summer, so I might try my hand at that if things go ok.