When putting together a costume or outfit, it's often the small, attention to detail items that really make a costume and also style it as being uniquely yours.
This is even more important if you bought your outfit because maybe someone else is wearing the exact same thing. So you want to add in your own accessories so you ensure that you definitely don't look the carbon copy of someone else.
What accessories would an Ice Queen wear? I'd expect her to look like she's got an icy, frosty or snowy exterior. Going with some snowflake accessories like a diamond or sparkling rhinestone pendant necklace or earrings would work well.
Drape yourself in jewelry and hair accessories that sparkle and shine. A tiara or a hair ornament that has the appearance of being made from beads of ice. Rhinestones, diamonds, white pearls, glitter, white feathers.
Another wonderful idea is to get hold of some lace and satin-like fingerless, long gloves which can truly transform a plain dress into something more graceful and looks great worn with or without a cape. The lace on these gloves is so delicate that it has the appearance of snowflakes.