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Be that boss. Challenge your employees in fun ways, challenge them to challenge each other in fun ways, challenge them to learn through personal projects, never be a dick.

Edit: And PAY them to learn through personal projects, if it has direct relevance to your business. At least some of it. "Oh, you bought a 3D printer and learned it in your own time, and now it's useful to my business? Can I buy you some upgrades?"
That's just investment. (Which my boss has basically done)