My rule of thumb is that if it is a popular trail and I will be gone for less than half an hour I just take a water bottle and the PSK items in my pockets. If it is not in a Phoenix park or I will be gone over an half an hour I take my PSK Camelbak.

I have lots of PSK items in my pockets:
ferro rod
Leatherman Charge Ti
AA LED
SureFire C2
Suunto Comet with glow in the dark compass
Fox 40 Micro
mini Bic
Fisher Space pen
two Coghlan''s Emergency Tinder in an Altoids strips tin
two band-aids
an REI Featherlite mirror cut to fit in wallet
three small sheets of Rite-in-the-Rain paper
list of important names and numbers on one sheet
a disposable glasses cleaner packet
a Fresnel lens

I once had a 36" x 36" sheet of aluminum foil and 10' of braided nylon cord but that seemed like too much. I'm not sure they would be useful on a desert trail.

Do you want an inventory of my Camelbak? <img src="/images/graemlins/blush.gif" alt="" />

If I was taking more than my PSK Camelbak, I would be backpacking not hiking. <img src="/images/graemlins/grin.gif" alt="" />