How you wound up with the leftovers: You're hooked on squirting lemon juice into your seltzer.
Use them now: Citrus peels are packed with nutrients as well as a surprising amount of flavor. Grate them and sprinkle the zest on green beans or asparagus. For dessert, simmer strips in simple syrup and roll in sugar; place on cooling racks to dry, about three to five hours. (They make a lovely garnish for pound cake.)
Use them later: Wrap the zest tightly in plastic and then seal it in a zip-top bag. Again, it helps if you mark the quantity (e.g., "zest of one lemon") on the bag. To defrost, let it sit at room temperature for a few minutes and then rub the zest between your fingers to fluff it up.