Flight activated LED's

It all depends on the size of the glider.

You could detect load on the wing and use that to turn on the lights, or perhaps have some flap held closed by aerodynamic forces (that will not be there when the glider is stopped)

Presuming the glider lands right way up, you could mount an LDR under it to detect when it goes dark (but then the lights would only work in the day). Also presuming it lands right side up, you could have a panel on the bottom of the glider that detects the weight of the glider (via a microswitch) -- kind of like a weight-on-wheels switch in large aircraft.