Use this as a checklist from the Ultimate Survival (and Enjoyment) Guide for the Obama Inauguration as you fill your pockets when you're heading out to enjoy the inauguration. Remember that you can't bring anything larger than a purse -- no backpacks or duffel bags are allowed. Put what you have in see-through plastic bags so they can be screened more quickly.

Maps
Bottled water (Vendors will be selling food and beverages, along with souvenirs, in the area surrounding the Inaugural Area.)
Remember, you cannot bring large bags with you inside the inaugural area, so you are limited in what you can carry. Travel light.
Money and identification
You will want to guard against pick-pockets by keeping your cash and ID in your front pocket.
Change, small bills, large bills hidden in case of emergency and put in various places on you
Credit cards (please call your credit card company and tell them you will be in DC so they do not stop your card inadvertently)
Metro cards for each member of your party, with enough fare for a round trip
Cameras (but no tripods or camera cases)
Extra batteries since batteries do not last in cold weather - keep them close to your body
Extra flash cards or film
Lens cleaner in case of rain, fog or fingerprints
Cell phones
Although phone companies are adding capacity for the inauguration, there's a strong chance that you won't be able to get a good signal in the Inaugural Area. Use text messages and e-mails instead.
Bring change for public phones.
A set of walkie-talkies, like the Motorola TalkAbout, might actually be a better way for you to keep in touch with other members of your group in case you get separated
A small, battery-powered, portable AM/FM radio
A poncho or raincoat in case of precipitation (remember, no umbrellas)
Comfortable shoes (you will be standing for hours, and then walking for miles afterward)
Most of the National Mall is unpaved, so if the ground is wet or there is rain, expect to have mud. You might want to find a comfortable pair of hiking boots to keep your feet warm and dry.
Warm clothing (best to dress in layers in case you get too warm)
Thermal underwear
Hat and gloves
Handkerchief or small towel in a plastic bag to dry your face in case of rain or snow
Extra pair of dry socks (wool are best) to keep in your pocket in case your feet get very cold
Hand warmers
Sunglasses
Hand sanitizer
Facial tissues (which can double as toilet paper in case the portable restrooms are out)
Lip balm
Enough medication for 24 hours in case of delays (diabetes, high blood pressure, etc.)
If you use contact lenses, you might want to bring some eye drops in case of windy weather
Bring adhesive bandages for blisters from walking
Aspirin, acetaminophen or ibuprofen
Something to settle your stomach, like Pepto-Bismol or Imodium tablets
Snacks (Bring enough for 12 hours with limited food)
Granola bars, trail mix
Fresh fruit and vegetables
Cheese
Sunscreen in case of bright sun and snow
Small flashlight (or headlamp) for walking in the dark
Portable music player with your favorite music (if you get a splitter and have two headphones you can share with your friends)
Small journal to record how you feel on this special day
Pen
Phone numbers of friends you want to send pictures to via SMS

Source: Ultimate Survival (and Enjoyment) Guide for the Obama Inauguration