While this park in Mclean, VA typically charges an entrance fee, it's completely worth it. Tripadvisor
I captured this image.
If staying overnight I recommend the relatively close Ritz Carlton Tysons Corner in standard season using a Marriott 35k point certificate for each night of your stay and points if needed in addition. That's where we stayed & how we stayed there for minimal cost. Another viable option is $ rates there if you have the Ritz Carlton Reserve card and want to capitalize on some free lounge access at a relatively low-cost Ritz on many weekends (while during the week it can be more costly). In some cases, prices can climb over $600, but in others, they can be found under $200.