This article actually covers a lot of what I was going to suggest. Note how it's almost completely language-agnostic, and that's a good thing. "Well Rounded" means you're looking for a very general outlook, a jack of all trades's view, of SWE. So you can't lock your subject matter into any one language, paradigm, platform, library, etc. That leaves you with the soft/technical skills of engineering as a whole, not just SWE.