I was going to suggest something like a small form factor PC running Plex or something like a Roku but these tiny NUC PCs seem to have filled a gap that is currently filled by a giant Antec self-build thing that looks like a full size AV receiver running an AMD 64 from 2009 or something

Then again that machine can run pretty full on (albeit old) games on Steam etc and has a disk drive so it's a bit more versatile than that. I'll probably never get rid of it even if I barely switch it on.