Lines, Shapes and Shadows. You don’t have to do all three in one day, especially if it isn’t sunny. This summer I did lines and shapes first, then shadows another day.
With little kids you can show images and ask what shape they see, to get them practicing their shapes. You can also introduce horizontal versus vertical lines as well. This is great to do in a classroom too, especially PK-K since there are so many toys and things in there.
I do encourage my students to play with editing, trying the different preset filters on the iPad. With the older students you can show them how to access the editing buttons and explain what saturation, contrast and what it means to adjust the shadows and highlights separately. In the past, with cameras, I switched them all to black and white mode and we photographed in that mode all year.