A commercial box, like NVidia shield, Chromecast, Roku, Amazon Fire stick or AppleTV. This is by far the easiest as you get a remote, a "10 foot user interface" and they work with online services like Youtube, Netflix, etc. If you care about everything being open source software this obviously isn't a good option.
You can also DIY your own box on a Raspberry Pi. Be prepared to do lots of tinkering, though.
The other option is to get the media server's UI remotely. Long HDMI cable, HDMI-over-cat5 or wireless HDMI gets audio/video; some of those also do USB, or you can do wireless control if it's close enough. Almost as much tinkering as a Raspberry Pi, but could be cheaper if it's not far.
Another big benefit of the first two options is it scales to multiple clients, if you have more than one TV.