Something I've tried to do more frequently with left over icing is make my own transfers.
By using your left overs, you can pre-make great embellishments to store, and use at a later date! Here, I've made some XL hearts to add to my Valentines cookies.
Simply print an image on line, or free hand your design. Use parchment paper (place printed image if that's what you've chosen-under the parchment paper), and squeeze your flood/medium consistency icing onto the parchment. Let it dry overnight, and the next day, they'll peel right off and be ready for use. Not ready to use immediately? That's OK! Store in an airtight container until you plan to embellish!