Off the top of my head, some things I've got:

-Cell phone charger
-Umbrella
-Spare set of clothes (socks/underwear/shirt/pants)
-Toiletry bag (razor/toothbrush/toothpaste/floss/hair brush/deodorant/q-tips/tweezers/spare pair glasses)
-roll of toilet paper
-Snack food (Lately, I like Quaker 'Oatmeal to Go' bars....I find them to be filling.)
-water bottle
-pocket AM/FM radio with ear-buds and spare battery(s)
-Multitool
-Bic lighter
-A good bright flashlight with spare batteries (I like Surefire's G2 LED)
-An N95 dust mask, work gloves, and safety glasses (my glasses are already safety glasses)
-A little roll duct tape
-Spare cash/change (Nothing crazy, just a few 1's, 5's, and quarters for tip's, vending machine's, making change, ect)
-A little pry bar (I like the Stanley Wonder Bar II)
-plastic silverwear
-Purell/hand sanitizer
-small writing pad/pens/sharpies
-some basic first-aid stuff (regular band-aids, larger sterile pads, advil, pepto-bismol, ect)

All that stuff fits into a regular day bag. The one thing I wish I could carry around, but don't because of bulk, is a nice fleece blanket. If I was in the same place all day, I would probably just keep one at my desk.

On a side note, depending on one's office atmosphere, it can be tough to wear sensible sturdy shoes. However, I really feel this is something everyone should make an effort to do. I don't like any footwear that makes it difficult to run.