I hate saying it, but Avatar looks amazing on my 3d set. There are multiple times during the movie that i thought something(bugs usually) was in my house.
As far as readily available, Resident Evil:Afterlife seems to get good reviews, all of the animated stuff seems to be reviewed well also. Ill be getting Harold & Kumar tomorrow, its supposed to be great.
Hubble 3d and Under the Sea are both very good. Sammy's Adventures is likely the best - but not a region A release (yet). Owls of Ga'Hoole has excellent 3D animation. I understand the not yet released A very Harold and Kumar Christmas has excellent 3D for a live action flick.
If I were just demonstrating, I'd probably pop in Hubble 3D because the content can be viewed by anyone and it's fairly short at 40 minutes and the 3D is excellent.
I still think Step Up 3D's dance off scenes have some of the best 3D reference material to date.
Based on reviews here and elsewhere , I bought StepUp 3D and was not that impressed at all. Sure while it shows depth, I didnt see anything noteworthy , with POP OUTS and real out odf this world 3D effects to showcase 3D.