This Swimming Pool Birthday Cake was for my twin sons' 6th birthday party, which was at our community pool. The cake is designed to look like that pool. I made it with 6-9x13 cakes (3 chocolate, 3 white) stacked side by side. The top layer was carved in the shape of the pool.
I used buttercream icing: white for the inside of the pool, brown/gray for the concrete, and green for the grass. I made light blue gelatin with 1/2 cup blue sports drink, 1 1/2 cups boiling water, 1 packet unflavored gelatin, and 1/4 cup sugar for the water. The handrails are white fondant rolled and shaped, then painted with silver dusting powder mixed with a drop of clear vanilla extract.
The lane lines and step details were made with candy melts (they don't melt under the gelatin like fondant does). The boys are made with fondant as well. The rope is cross-stitch floss, twisted and coated in corn syrup to keep it from unraveling (the only inedible part). The beads on the rope are fondant and I waited until the last minute to put them on the gelatin because they melt in contact with it.
The noodles (floatation toys) are made from 2 taffy candies each, softened in the microwave and rolled out. The tiles around the pool are fondant that I rolled out and cut. So are the depth tiles, I wrote on them with food coloring marker. I made sure the gelatin wasn't very deep so that it wouldn't come in contact with the tiles and melt them. The diving board is white chocolate-covered graham crackers, cut and stacked, with a pretzel stick stuck through the base to give it extra support when put in the cake.
The beach ball is candy clay (Wilton recipe with candy melts and corn syrup) painted with Wilton's candy food color (oil based). The tree is a pirouette cookie stuck into a ball of popcorn covered with green candy melts. I put the tree in too early and it absorbed moisture from the cake and then fell over, otherwise it would have been taller in the photo.