If you’re looking for a cafe near the monuments in D.C., there is one in the Smithsonian Castle, but do NOT recommend. It is both pricey AND very limited in its selection (cold sandwiches, chips, lots of sweets); we regretted our choice to get food there, as none of us enjoyed it much and didn’t even finish what we got. Even the bottled water tasted bad to all three of us!
What we *should* have done is grabbed food at one of the many food trucks (offering various ethnic cuisines) right outside the Washington monument, and then walked a block or two over, brought said food into the Castle, to sit and eat comfortably at a table out of wind, where we could also peruse one of their maps, browse the gift shop, use the restrooms (instead of resorting to the port-a-johns outside the Washington Monument), etc.
The kids did think the Castle was cool, though there isn’t really anything to see there other than the architecture pictured, and a rose garden. It used to house all the exhibits, back in the day, but it’s basically offices and a visitor center for the Smithsonian now.