Car computer. Run mpd and huge stash of music on it, playing through stereo, also streaming to tablet, and functioning as NAS for tablet (start a batch of transcoded movies transfering from car-computer to tablet when you start your drive, they're on by the end).
Don't know how good Linux BT support is in general (I know HFP isn't really working on tablets, but maybe better on desktop Linux), but if you could make it talk HFP to your cell-phone, and stream to/from your tablet by wi-fi... All sorts of options.
I've an old laptop, with good display, that I'm going to do such stuff with when I've time.