I'd suggest incorporating leftover spaetzle into a stir-fry or soup for a delicious and creative way to enjoy it again! Though, remember to adjust the seasoning and cooking time accordingly to avoid overcooking!
The best way to reheat spaetzle is to sauté them in a skillet with a little butter or oil over medium heat until they are warmed through. You can also sprinkle them with a little water or broth to prevent them from drying out. Be sure to stir them occasionally to ensure even heating.