Tweezers, a small Phillips screwdriver and something to open the device. (I used the iFixit Toolkit)

1 piece of paper (regular printer paper, post-its, ...)

Quote: (Edit)

Shortcut: It is also possible to improve the buttons without removing the whole button frame. See post #3 for details.

I got one of the very early Nexus 9. My device was mostly fine (no huge light-bleeding) but the Power and the Volume-Up buttons were hard to press because they were not raised high enough above the surface.(Volume-Down was working fine)If you follow my instructions you do this at you own risk. I'm not responsible for any damage you do to your device.Take of the back cover of the device. ( https://www.ifixit.com/Teardown/Nexus+9+Teardown/31425 Remove the 3 screws (red) and the big connector (green).Pull out the metal body holding the buttons. Pull carefully on the button cable to remove it from the connector.If you look closely you can see that the Volume-Down button is not completely attached to the metal frame. This looks like a production error but it allows the button to be pressed normally.We could apply this "production error" to the other buttons too which should improve their usability.First separate the buttons from the metal frame. I found that only a piece of paper is thin enough to not cause damage to the buttons.Now, use the same piece paper (or another one - I don't care) to create tiny pieces that can be put behind the buttons to raise them a bit. (I folded the paper once to have 2 layers)Carefully put everything back together.All buttons work fine.