I will 2nd Carnation Cafe - Potato soup is usually very good (though last time I was there it was lukewarm and less good, but still good)
I also REALLY like River Belle Terrace. They've started serving brunch since the last time I was there, so I've not had it. We always did breakfast there at least once each trip before they stopped. It is also good for Dinner, but as it was one of Walt's favorite breakfast hangouts it has some mystique for me in the morning.
Additionally breakfast Ronto Wraps are surprisingly good.
Rancho del Zocalo is another of our family's favorites, though I haven't been there since pre-covid.
In DCA, Pym's is pretty good for a quick bite.
I also feel like the Wine Country Trattoria is really under-rated. It's always easy to get a list minute reservation and I've never been disappointed with the food and drinks there. Maybe a little over-priced for Italian food, but the service has always been great!
I also REALLY like Flo's Diner. The fried chicken always leaves me happy, and the 50's diner ambiance (right down to the blue plates) coupled with the various sight gags really speak to me. We typically do at least one meal here per trip as well.
And I'll second Lamplight Lounge, be it for brunch or any meal really (or even just for afternoon or evening drinks). It's so cool to be chillin' down by the water enjoying tasty food and drinking in the views (and beverages).
Have a great trip!