If your inner child is sitting on a beach somewhere, bring one of those zen gardens with "sand" or rocks or whatever and rearrange them? I'd bring "box" of damp sand and some molds and make a sand castle? Find a coloring book with beach scenes?

Why is your child on a beach? What's the association that you put her there? My grandparents had a beach cottage we'd go stay at and there's all sorts of "beach" associations I have to it and directions I could go with that one :-) Maybe bring one of those child "magnetic" fishing games where you literally fish? Think of your "hobbies"/interests now as an adult and figure out how a child would experience them for the first time. You didn't spring full-grown into your adulthood/interests/skills. Pick something and go find out what the child version is?