City area caches...I usually don't. More rural caches that might involve a short hike, I'll carry a small canvas surplus pack filled with a couple bottles of water and geocaching items...batteries, headlamp for night searches, trade items, etc. Maybe throw in some bandaids.
Now, if the cache is in the mountains, I'll go to my regular day pack...3500 cubic inches and approx. 25-30 lbs. First aide, extra clothes, raingear...yes, even on sunny days...storms in mountains can come from nowhere, extra food, water (5 bottles), water purifier, caching trade items, batteries, emergency/overnight stuff, personal items, map and compass, knife, headlamp,etc. I also leave a note as to where I'm going and also usually list one or two possible plan changes. The day you don't bring all this stuff...you'll need it!