For years our in-laws took us to Rehobeth Beath/Dewey Beach/Bethany Beach. It’s a good beach for castle making and body surfing, there’s a nice little board walk in Rehobeth with Funland for the kids. If you pick the right week there’s a really nice outdoor art and craft show in Rehobeth. (Aug 15 and 16 this year). The Rebotheth sandcastle contest is fun too: https://www.beach-fun.com/sandcastle-contest.html North of Rehobeth Henlopen Acres State park is worth a visit as is the Delaware Sea Shore to the south.