Wow, I never realized there were so many lite-packers on the boards.
On an average day to the parks carry;
Lowepro Micro Trekker 200 camera backpack containing;
Nikon D100 w\ 24-120mm Nikkor
SB-80DX Speedlite
(4) 1GB Lexar CF cards
105mm Nikkor Micro
2 spare batteries
Various filters, lense cloths, blower brush, remote shutter release, etc
TRV-33 MiniDV camcorder
Casio S500 mini-digital
iPod w\ photo adapter
Depending on what kind of shooting i'm doing that day I also carry a small Velbon Maxi tripod or I may lug big Bogen out and store it in a locker until evening.
Since we usually go in January \ February, I usually roll up 2 long sleeves, one for myself and one for my better half for the evenings and throw that in the bag.
Otherwise, resort charge \ key, photo ID, $~10 cash and some change in a waterproof pouch. I throw all of my receipts in the pouch as well as it helps us plan new vacations, giving us an idea of what we spent on food, souviners, etc.
Most of the gear is obviously because of photography, otherwise I can get away without taking anything except my S500 digital.