If traffic is not available any other moving assets (ships, trains etc) will do for the demo too.

I've done a bit of googling, but that hasn't turned up anything useful that's truly open and free. This might be worth looking into: http://www.ns.nl/reisinformatie/ns-api but that's showing departure times, rather than train locations.

There's various ship- and plane-tracking sites, but they don't seem to offer a feed for free.

Maybe just get one of those cheap GPS trackers and let a colleague drive around for a bit during your demo?

Try AIS (Ships) and NextBus (Buses). It won't necessarily be a WFS service, but if you have access to FME Server then you could very easily turn one of these into a WFS feed and use it as a source.

Also look through the knowledge centre for realtime FME Server demos. There's one that uses content from San Francisco, with buses, planes, ships - I just don't know if it's WFS

@erik_jan - I was also pointed to this resource recently. It seems like it should be very helpful to you!

http://directory.spatineo.com/