The below managed to feed about 25 kids and 10 or so adults. At the party, each kid was allowed two out of the three elements since I didn't have 30 squares worth of each element, and it was great fun to have kids come up and request "2 dirts" or "1 sand, 1 water."
Sorry I didn't have any "making of" photos. Basically, the cake was such a hit, I got a lot of requests to post it, so why not on Instructables?
Step One: Make Water
3 Packages of Blue Jello
3 Cups Boiling Water
3 Cups Room Temperature Water
8x8 baking pan
(1) Follow directions on box but make at double strength (use half the water)
(2) Use to fill 8x8 pan to create jello at least 1.5" high
(3) Refrigerate overnight
(4) Before assembling, cut into 2x2 inch squares
Note: Water was a BIG hit. Most kids don't get jello birthday cakes. I wish I had make twice as much!
Step Two: Make Sand/Tree
1 16 oz box of Rice Crispies, split into 3 bowls: 1 8oz, 1 6 oz, 1 2 oz (for tree)
2 sticks of butter, split into 1 stick, 3/4 stick, 1/4 stick (for tree)
2 bags of marshmallows, split into 1 bag, 3/4 bag, 1/4 bag (for tree)
1 small bottle of chocolate sprinkles
green food coloring
Twix Candy Bar
Baking spray or butter to grease baking pans
2 8x8 square baking pans
(1) Spray baking pans with butter, or grease with oil
(2) Melt 1 stick butter in large pot. When melted, turn off heat, add marshmallows (if you leave heat on, rice crispies will be hard as rocks). Marshmallows should melt with residual heat.
(3) When marshmallows are melted, add rice crispies, and chocolate sprinkles. Mix.
(4) Spread onto one 8x8 baking sheet
(5) Repeat steps 1-3 with 3/4 stick butter, 3/4 bag marshmallows, 6 oz rice crispies
(6) Spread onto 3/4 of a 8x8 baking sheet. Use parchment paper, wax paper to separate these rice crispies from rest of pan.
(7) Repeat steps 1-2 with 1/4 stick butter, 1/4 bag marshmallows
(8) Add green food coloring to melted marshmallows. When the right color, add rice crispies.
(9) Spread onto the last 1/4 of a 8x8 baking sheet
(10) While rice crispies are still warm, put flat side of Twix bar together and press into green rice crispie square. This is the tree trunk. Much stabler and easier to do before the rice crispies harden!
(11) When cool, cut into 2x2 inch squares, except for the tree. You might want to trim the tree leaves so it isn't too top heavy.
Step 3: Make Dirt
3 boxes of brownie mix (although I made from homemade - google smittenkitchen or America's test kitchen for easy recipe)
One small bag of mini chocolate chips (or use brownie mix that has mini chocolate chips in it)
Cooking spray or butter for greasing pan
9 x 13 baking pan
(1) Make a parchment paper sling, so it's to pull out brownies from pan later. Spray or grease sling in baking pan
(2) Make brownies per directions
(3) Put in baking pan
(4) Bake per directions
(5) When cool, cut into 2 inch squares
Step 4: 3D print a Creeper
(1) Download Creeper from Thingiverse and Print: http://www.thingiverse.com/thing:11825
(2) I didn't have green plastic, so I color printed, cut out and covered the ugly plastic with this, although I had to fiddle with shrinking the size so it fit the Creeper: http://pixelpapercraft.com/papercraft/4f1b1e1a6f923f8035000002/creeper
Step 5: Assemble
Green cake icing
(1) Cover baking sheet with aluminum foil
(2) Give results of step 1, 2, 3, 4, 5 to 8 year old
(3) Watch him assemble the creation
(4) Frost top layer of dirt with green icing
(5) Take photograph