Just Sunday I was invisible to my Step Mother of all people. I let her pass, then I pulled up next to her. When finally noticed me she started yelling at me.. about how I could hit her truck. So when she slowed down I just went past her but I made sure to lightly hit my knucles on the bed of the truck, which made a loud thump... scared the crap out of her.

As for a cape, try a superhero surplus store.