Accelerometer switch to trigger LED fade
There is a form of accelerometer that is commonly seen on kids toys, is super cheap, and works reasonably well when calibration isn't critical.  It's made from a spring and a metal post...  

Imagine this:  start with a simple spring like what you'd find in a ball-point pen.  Inside the spring is a simple metal post (or a piece of solid-core wire without insulation).  The post and spring are mounted together on one end and the other end flops in the breeze.  Normally, the spring and post do not make electrical contact.

When this is struck (or, what it's mounted to is struck) the spring with deflect and contact the post, thus making electrical contact.  It only responds to force perpendicular to the post, so you might need a second one to get "3d response".  Vary the spring and post to change the sensitivity.

Here's a photo of one I just whipped up in the lab:  

Ignore the other stuff on the PCB.  I took a stiff piece of wire (not stiff enough, IMHO) and soldered it down.  Then I took a spring from a pen, unwound a little bit of it and soldered it down.  Then bend whatever you need to in order to get the wire centered on the spring.