Here is a great winter theme App that I like to use with little ones who are learning to spell. This App provides letters in mixed up order and asked children to spell specific words using the letters.
To turn this into an OT session, I ask children to WRITE the words that they get correctly on a whiteboard. This way, I work on handwriting. Also note, I always use a stylus (with a gripper on it) during all iPad games so that children practice correct grasp on writing utensils.