Elsa has a blonde French braid and a long, icy blue dress. Wear the appropriate wig or style your hair in the right way and you will have a lot of people saying that you look like Elsa from Frozen, especially since this is a movie with such enduring popularity. A long and slinky blue dress can certainly work.
Elsa costumes are easy to find these days. However, some people might appreciate the creative challenge of making their own. Pick out an ice blue dress, add a silvery tiara to the ensemble and even better if you can find giant plastic icicles or anything that resembles icicles, adding those as motifs can really help to make an Elsa costume stand out, since she is literally the Ice Queen. This will add a touch of originality to the Elsa costume.