Lady Yukari's powers for one day?
I'd prolly ask her permission to grant myself control over the Boundary of Imagination and Reality...with some limitations, and then gap myself all over the world, experiencing and enjoying as much as I can before gapping into Gensokyo to settle there. Add to that a few tweaks to my health and fitness levels; another to my brain for a proper memory, perfect understanding of ALL languages; one to all my senses to fix them up to human limits with the added bonus of being able to perceive and touch spirits; and lastly one final tweak to make sure I can never, ever get drunk.
Beyond using the powers for myself, I won't do much help anyone else to be honest, not unless they attract my attention somehow, there's just too much that could either go wrong (especially if I get it into my head that I could help everyone)...plus, I'm firm believer in earning salvation.
The last thing I'd do before leaving is to make all traces (records, my stuff, memories...that jazz) of my existence are gone or come with me (some of my stuff), and that no one remembers me.