If you keep hearing 'toy box only' and don't understand let me quickly catch you up... the Disney Infinity game has two modes. Toybox mode and playset mode (in which you need to put one of those fancy looking crystal pieces on the base). The playset pieces are actually a game of their own. In the starter pack there is a Monsters Inc, Incredibles and Pirates one. Only characters corresponding with that world will work in it... so for example, Sully from Monsters Inc can't be played in the Incredibles game, only Mr Incredible, Violet, Dash, Mrs Incredible and Syndrome will work in the Incredibles game.
Toybox mode is where you build your own game... possibilities are endless e.g You could make a town featuring pride rock, Cinderella's castle and the apartment block from Wreck it Ralph... and stick a car in to race around. Within toy box mode there are lots of mini games to play and EVERY infinity character will have its own mini game to access in toy box mode.
Elsa is a toy box only character so there is no Frozen game to play her in. Even without a game though she is an awesome character for the toy box. She is very strong and in the battle arena games can get very far from a low level with her ice powers alone. It's also very cool visually to watch her freeze characters as well as enemies. Anna didn't fare as well for me because all she has is a climbing hook which I found quite boring.
Elsa's mini game is a slingshot game, firing as many infinity balls as you can in the time limit. Found this a little random and not a great match to the character but her special powers and strength in the other games made up for this.
Elsa is also voiced by Idina Menzel who also provided the voice of Elsa in the movie. If you leave the character alone for a while she'll mutter a few things to you.