I would fly above the trees.
I would write a novel.
I would cook more often.
I know I could try to write a novel, but I never get past the brainstorming stage. When I look back at my plot ideas, they sound silly, so I ignore them.
And I am not intuitive AT ALL in cooking. I've wasted so many ingredients by failing to put a lid on the pot (the instructions didn't say to) or not knowing the proper amount of a "sprig" of rosemary. I'd try all sorts of inventive cooking methods if I knew they wouldn't fail.
Sure, I could take lessons. And I have, in other things. But it's fun to suppose, isn't it? What would you do if you knew you could not fail?