What I carry on a 2-4 hour excursion varies greatly depending on where I'm going and what I plan to concentrate on shooting. If I'm just out for birds, I'll probably just do like Scott and take the camera with my long lens, *maybe* my 18-55 kit lens in case there's a landscape just begging for attention.
In that case, I just wear a fanny pack--kit lens in one of the two "cup holders" on the sides, two water bottles (if it's warm out, I freeze at least one of the water bottles so I'll have cold water for hours) in the main compartment, along with a couple of plastic bags and whatever other essentials.
If I think I might be doing some macro work, or simply have no idea--or if the weather or terrain dictates more gear (i.e., I need a rain jacket, or somewhere to put a light jacket if it gets too warm, or extra socks, etc), then I just take a regular backpack. Extra lenses go in a padded outside compartment, jackets, socks, first aid kit, extra water, snacks, etc in main compartment and I usually still take the fanny pack too, since I gives me much quicker access to the lenses I think I'll want most.
I rarely carry my tripod--mostly because I have yet to get a good one that doesn't weigh a ton. I sometimes take my monopod, which I either use as a walking stick or attach to the backpack.
EDIT: Oh, and sometimes I also carry a small, lightweight camouflage blanket that I can spread on the ground or wrap up in--especially in the winter!