Jaimie Alexander plays Lady Sif in the Thor movies. She recently visited the Children's Hospital Los Angeles, but instead of going as Jaimie Alexander, she went as Lady Sif. Sword, armour, the works.
OK, so maybe it's not quite as cool as having Thor himself turn up, but it's still pretty damn cool. Especially since that sword looks kinda real.
Lady Sif Visits the Children's Hospital Los Angeles [Marvel, via Super Punch]