A small sledge hammer.

Useful to adjust the perfomance of your competitor's airfoil or help
drive in tie-down stakes.
You should have a tiedown kit with straps, ropes and a hammer etc. Even
if you don't plan to tie it out, there is always a chance of a fast
developing storm that approaches before you can find 2 other guys that
aren't trying to disassmeble their gliders before the gusts hit.

Also in the trailer be sure you have
fuses
lug wrench for the trailer
not bad to have spare glider tubes

Chris