Here's a project I started on and threw together a couple of years ago and then relegated to my pile of half completed projects. Just unpacked them after a move and will hopefully get them at least playable soon.
The blue unit is going to be a Super Mario Bros. theme, with a clone NES inside.
The green unit is going to get Frogger treatment and run the "TV Games" style Frogger handheld electronics.
I haven't decided what the white unit will be at this point.
All three will have 5" LCD screens; so far I only have one, from a Gamecube add-on monitor. I don't really know what I'm going to use for controls yet. There seem to be some decent options for small buttons, but I've not seen a small joystick I'd be happy with.
Obviously the intent is more for show than go with these, but I'd still like them to be relatively playable.