My UX prototypes feel 'sterile' – how can I inject more personality and user delight?

I've been working on a new app's user interface, and while it's perfectly functional, the prototypes just feel too generic and sterile. I'm really struggling to add that 'spark' and personality without compromising usability or making it look unprofessional. Any specific techniques or strategies you use to make your UX designs more engaging and delightful?