Bottled water and a picnic lunch. Transportation will take 5x longer than you think, food is inconvenient and super expensive near the Smithsonian, and an exhausting day will be better if you don't get hungry and dehydrated.
Traveling in and around the DC area in a scooter is very doable. If you are prepared. Check and double check the weather each morning on a local radio channel. Be especially prepared for rain and the sun. Riding the metro with a scooter in tow can be an issue. Not all stations have operating elevators. You can check the daily status of the elevators via the DC Metro's web page. Though buses do take on wheel chair patrons I am not sure about scooters. Again check the metro web site. Most importantly is to have a battery charger or a backup battery for the scooter. Make the traveling easier by going during non-rush hours. Good luck and enjoy yourselves.