As my daughter, Jolie, has a summer birthday, we always have to plan something around keeping cool. For her 7th birthday, she wanted to have a large blowup water-slide in the backyard (the kind you rent), so she wanted a Waterslide Cake Design to match. I came up with the design using different objects and trial and error. We vacation each year 2 weeks before her birthday, so I decide to do a trial run cake during that time.
I baked the base cake using 2 cake mix batters in a 11x15 pan. Then I baked another mix using a 9x13 pan. I froze all the cake as I baked them the week before. While they were still frozen, I cut off about 1/3 of the 9x13 cake, and the proceed to cut additional pieces to make a 3 pieces. I stacked them and secured them to one corner of the larger cake using wooden skewers along one edge of the stacked cakes, leaving about 2" border on the long side and about 4" on the short side of the large cake. I proceed to carver out a slide. I have found that carving a semi frozen cake is less messy and it just seems to carve easier.
At the bottom of the slide, I carved/dug out a "pool" in the larger cake. I frosted the slide and pool using a blue buttercream, and the rest in a green buttercream frosting. I added edible blue glitter to make the water shimmer.
Using a small paint brush, I painted swimsuits on Teddy Grahams and proceed to decorate the cake. I used large gummy Peach rings (gummy lifesavers were too small) as water rings. I cut out beach towels and rafts from fruit roll ups. I used Twizzlers pull-a-parts to create a ladder and noodles for the swimmers. I found large yellow and white striped lemon balls (similar to peppermint melting balls). I used paper drink umbrellas as beach umbrellas.
She was totally amazed as were our guests. She was so proud of the cake and enjoyed showing it off to her friends as everyone arrived to her party.