To describe a person or a solution that takes a realistic approach, consider the adjective pragmatic. The four-year-old who wants a unicorn for her birthday isn't being very pragmatic.
The opposite of idealistic is pragmatic, a word that describes a philosophy of "doing what works best." From Greek pragma, "deed," the word has historically described philosophers and politicians who were concerned more with real-world application of ideas than with abstract notions. A pragmatic person is sensible, grounded, and practical — and doesn't expect a birthday celebration filled with magical creatures.