Played two levels of Thumper, some of Superhypercube... those are both excellent. Definitely recommend those.
I also played the Tumble and Hatsune Miku: VR Future Live demos. Tumble was pretty cool, but not enough for me to buy it. Same for Miku, which loses its charm once that demo's song is done.
The free Allumette short animated film is a great watch. If you have your play area set far enough back, you can walk around and look/around into the sets.
The Within and Littlstar VR Cinema apps have some interesting short films and VR clips to check out. Mr. Robot, Paranormal Activity, Mythbusters, etc... worth killing a little time in those.