Pirate - stripy shirt, 3/4 pants (or cut the bottom off an old full length pair), scarf round your head and voila!
Princess - cheap $2 store tiara, glitter galore, shiny dress or skirt and tank top
Cat - black leggings, a stocking tail, black gloves and mock up some ears and draw on whiskers
Geisha - silky dressing gown, socks and thongs and makeup
Rockstar - boofy hair, leather (or fake pants), boots and a fake guitar
Thats all I can think of for now, a theme would help