Hey there @Woodsprite ! Local hikes in the DC area are my fave! My partner and I added a dog to our family last year so we also actively look for dog-friendly spots. If you are in the District, Kenilworth Park & Aquatic Gardens is a great spot - and dog friendly (not all NPS areas are!) This is a super easy hike (more like a stroll) and beautiful DC spot. It's not far from Union Market for your lunch nosh afterward.
I frequent Huntley Meadows Park just south of Alexandria, VA. It's also easy - totally flat, and has a really cool boardwalk over a protected marsh land. There are some incredible seasonal changes you can see as well as birds galore. Early morning is the best time to visit. the boardwalk portion of the park is off-limits to dogs, but the rest of the park is open! There aren't a ton of lunch spots directly near here, but Huntley Meadows isn't far from Old Town Alexandria, which is full of great spots
If you haven't already, you should also check out Manassas National Battlefield Park . It's a little past Fairfax and a short drive from DC proper, but wide open fields and trails totally dog friendly. This park is interesting as the location of the first battle of the Civil War. It's also a national park site (that is dog friendly). This is a good spot to bring a picnic lunch!
I had some really nice hikes in Cabin John Regional Park last fall while waiting for my daughter who was visiting friends for the day in nearby Bethesda. Trails were pretty easy, varied and sometimes along rivers or streams. Maps are available at nature center parking lot. Not too many people, but at least some others passing once in a while. Trails have loops and shortcuts so you can hike whatever distance you like. I didn't bring the dogs, but I'm pretty sure I remember seeing some on the trails