I sometimes build benches/chairs/arbours (normally made from willow or alder because the branches bend well without snapping if they're kept soaking wet). And yes, I'm well aware I missed the hippie era by a few years and have been making up for that ever since lol
But apple could work too for this purpose (assuming all of it is not as dead as a doornail). Not sure about planters though. My apple tree is 25 years old and it would have to be a tall skinny planter lol. Might as well saw it off well above the height you want your piece to end up at and see what you've got. Mehaul is quite correct about core rot being very common, but you never know til you open it up. Not cutting it toooo far down allows you some creative license as far as carving/sculpting goes.
If all else fails, apple not only smells great in a woodburning fireplace, but is a perfect wood for "smokers" or other ways of cooking meats, particularly fish and pork grilling. Yes I have recipes : )))
As much as I love my friends, I won't jump off a bridge WITH them. Instead, I think it's in our mutual interest for one of us to try to catch the other when they fall.