This Homemade Sea Turtle Reef Cake was for my son's 1st Birthday. The theme for his nursery and the party was Sea Turtles and Ocean Reef. Everything is edible; using a combination of fondant, rice crispy treats and crusting buttercream.
I use a regular box mix and always add a box of instant pudding mix to my cakes (the best for moisture and creamy flavor) and it adds enough moisture that you can bake the cakes the day prior, frost them, and still have that fresh baked taste the following day.
I prefer to only use fondant for decorations and use a crusting buttercream which, after frosting, can be smoothed down using a non-print paper towel which is how I achieved the smoothness on the tops of the mushroom caps and then left the edges rough like a mushroom can be. Decorations will are less likely to be eaten can be make a week or two in advance and kept in a container. In this cake, I made several elements in advance and allowed them to air dry so they would be plenty firm. For example, the octopus, shells, crab, and jellyfish were made two weeks ahead and dried for 3 days, then I was able to use food coloring to watercolor some extra texture and detail. The turtle body and shell is made from rice crispy treat covered in fondant and then also watercolored for extra texture and depth of color. Make the coral in advance and allow to dry firm, then spritz down and dust with sugar to get that crystal texture. Natural or Brown sugar are great for sand!
Lastly, to best get the fish to "free float" I used a toothpick to help stick them to the cake. I was questioned once from someone I would refer to as a cake "purist" about using toothpicks as "cheating", but I graciously reminded them that dowels are used all the time to support cakes, why should toothpicks be any different! If, however, you prefer to maintain purity, you can make hard sugar candy "toothpicks" if you have the time.