If you make it brackish, you could keep crabs.
If you're staying freshwater, I think 15g is a little too small for a crayfish (I seem to recall they like at least 20g). Or you could keep some frogs. African Dwarfs are easy to keep, as are (much larger) african clawed frogs. You could think about an axolotl too, I think 15g is enough for one of them...