I wouldn't send anything, as it'd break peoples minds. Whatever you send, it'll be something that didn't exist back then, else there'd be no point sending it. That means the younger "you" would have the modern parts. If the younger "you" had these parts, and "you" are you, but younger, how come you (not "you") didn't have them? However - if I was forced to send something, I'd send a note saying: "What are you so scared of?", a big wad of cash, and a number for a highly successful phsyciatrist.