Greenbrier state Park - near Frederick. Has a lake and beach.

Cunningham Falls has a beach but get there early on the weekend - or go during the week because it fills up.

Lake Arrowhead in VA is 2 hours from me in downtown DC. It's in a large park with a playground, benches, grills, etc. so it's easy to make a day of it. There's port-a-potties. The beach area is great, but not very big, so I would get there pretty early if I were you because it does get more crowded than I am personally comfortable with as the day goes on. My 6 and 4 year olds were in heaven there for their first (and only so far) swims this summer.

We went to Patapsco State park in MD last weekend- it's like 45-60 minutes away from downtown DC and it's near Ellicott City. There is a creek and people were wading/swimming in it. It was nice. The part where we went down to the creek is an extremely steep hike, my 3.5 year old did it herself (both down and up) but if I were to go again I would look for other entry points along that way. We arrived at 9. By the time we left at 11 the park was closed as it was completely full- so go early.

I got the recommendation from that website kidfriendlydc.com you might check there as I think they had other options too.

Looking for some day trips with our 6 year old son and 4 year old daughter. Prefer someplace we can swim, bring food or grill that is no mote than 1.5-2 hours away to beat the heat. We’ve been to Sandy Point and are looking for other options if they exist.