Great Falls National Park in Virginia is about 30 minutes from Washington DC, and honestly it's become my go-to hike whenever I want to be near water without driving far. The trail runs right along the Potomac River, past the falls, and it's both beautiful and genuinely fun to walk — not just a "nice view, then done" kind of place.
On my most recent visit the falls were running really full (there'd been rain the week before), and there were volunteers and staff out on the trail helping people figure out routes — happy to point us toward other hikes and parks in the DC area too. Little things like that make the park feel well looked after.
It had rained a few days earlier, so I actually spotted a bunch of chicken of the woods mushrooms growing on the trees, and the mountain laurel was blooming too. If you time it right after rain, you get a completely different park than on a dry week.