Have the diaper bag and stroller ready to go as this is the home base for the day. Bring a water bottle and snacks for your nugget. If you prefer to take public transit, google transit instructions and save them on your phone. Regardless, wear comfy and supportive footwear.
A stroller is the best option for carrying your nugget on this mini-adventure (a hiking carrier is the next best choice).
The breakfast joint gets pretty busy on weekends. Try to be there before 9 a.m. if you are doing this itinerary on a Saturday or Sunday. The National Gallery has discounts on certain days of the week.
This day out is most enjoyable on sunny days anywhere between May and October. If you plan to visit during the winter, skating on the canal is a great way to get from point A to point B. The canal is only open during the coldest time of year (January and February), so additional prep beforehand (winter gear, layers, extra clothes) is a good idea. You can also bring strollers on the canal.
Save money on admission to the National Gallery by viewing it's website for the discounted days.