What do you think is essential to pack for a day in the parks shooting

This is what I had all days, for better or worse:

Tripod
Extra memory card
Extra battery
50mm 1.4 lens
18-200 lens
Both lenses had UV filters
Shutter release
Lens Pen
Microfibre clothe
iPod Touch (the Stopwatch came in really handy for bulb shots)
Nintendo DS

All inside my handy Sling Pro, which I managed to attach my tripod to.. The only gear I found useless was the Gorilla Pod, because it was too much hassle to wrap it around things, and it proved easier to set up the full sized tripod.

And I second Justin's advice about the quick release.

 

Just to add to all the great advice...after my last two trips I will always have a spare camera. In October my dslr was accidentally ripped out of my hand and slammed into the pavement. The lens took must of the punishment and I was fortunately able to use it the rest of the trip (it was just the second day of our trip). On our last trip in January I foolishly had just one battery and left it in the room one day. I had to decide between spending the morning with my family or running back to the room. I decided to stay and although I missed some pictures I have the memories! As soon as I got home I started looking for a good point and shoot and found a mint G7. It doesn't soot in raw, but does everything else I want in a p&s and the controls are similar to my 40D. It was a steal at $250 with an extra battery and only 300 shots taken.