The latest puck-size black Apple TV is small enough to hide away or even mount behind a flat panel. But since I keep our unit out for improved remote reception, I like this idea from Lazerwood which adds a small decorative/decor-friendly touch, a wood veneer appliqué that gives the media device a little midcentury style.
The stick and peel laser cut piece of wood is available in walnut or black stained cherry finish for $20. I'd be the first to point out your Apple TV doesn't need a "cover" like an iPhone or iPad, but in a living room environment (especially one with a lot of wood furnishings) the Lazerwood appliqué could warm up what is essential a shiny dark piece of plastic (a beautifully engineered piece of plastic and electronics, mind you).
A budget option is to turn to that always popular and affordable DIY material of choice: contact paper! Same idea as Jonathan's desk and IKEA shelving, but on a much smaller and manageable scale; just trace, cut and apply. And the nice thing about this option is you can easily remove it if you feel like changing up the color/finish.