Remove the straw at the end of the trial as part of your cleanup. Don't leave the towels where the wind can flap them and attract the dogs. These should be waterproof and large enough to fit a hand with writing implement. Lots of variety is good. Bring rocks, extra clipboards, binder clips, and large envelopes.
Bright sun can cause stopwatches to become difficult to read.
Bring extra stopwatches and when one gets so faded that it's useless, get another and put the first in the shade. That first one can probably be used again later in the trial.