When my son was in a self contained class I made a "pyramid treasure" cake. I baked a cake in a jelly roll pan. Cut it out into graduated squares (one was 7X7, next 5X5, then 3X3, then 1X1. I used a basic white cake, and homemade icing (my kids reacted more to the food color than the sugar, as did the other kids in the class). I put the 7x7 square on cardboard and cut out an area in the middle that would be covered by the 5x5 piece. I cut out maybe a 2x2 section of the 5x5 piece, so the hole was 2 layers deep. I used thin icing layers to put the cake together, put small toys in the center for "treasure" and then just stacked the layers. It looked like a Mayan pyramid. I used a sprinkling of ghirardelli ground sweet chocolate and cocoa (same texture as cocoa, but sweeter - what they have in the shakers at coffee houses frequently). B.c I used thin layers of uncolored icing, and a dusting of cocoa to resemble dirt, it had no food color. it also was fairly low in sugar.
I also brought in queso and chips. The kids each ate a piece of cake, but the double batch of queso was devoured - NONE left.
Family Fun had a neat cake made out of fruit - slices of watermelon mainly, in their back to school issue. You can look online to find it.