You need some sort of aireation and water flow. You don't necessarily need a filter. A small powerhead near the surface to cause a steady surface current should do. Also, I would go with a sand bed and live rock, which will act as a biological filter. A hang on refugium would be cool, but is not a necessity.
You saying you have a 5 gallon tank as your display? If so, check out Nano-Reef.com for more specific advice. Is this your first saltwater tank? If so, I'd go way bigger than a 5 gallon. At least a 20 or 30gal, preferably a 55 or 75. Not only will these larger tanks allow you to keep more fish, but they're also provide a much more stable environment. JMO.
But as brandon said, get some live rock and water movement at least. No skimmer necessary as long as you religiously perform a 10-15% weekly water change.
