I made this rubber ducky cake for my son's first birthday - he loves his rubber ducky!
I had about 30 guests and needed a big cake. The bottom tier is 12 inches, middle 9" and top 6". I used store bought cake mixes and canned "pure white" icing.
Each layer was stacked using cake boards. I didn't need to move the cake much, so I didn't do anything to prevent sliding. I iced all three tiers, then used a round tip to add large "bubbles" around all of the edges, and to cover the edges of cake boards that were exposed.
I used a plastic rubber ducky (from the baby section of Target) and royal icing to make the topper. I made both blue and white royal icing. I reserved some blue to make the decorations for the rest of the cake, and to the stuff I used for the topper, I added a tiny bit of extra water. The thinner mixture dries shiny - so it looks more like water. It is not nearly as hard when dry as other royal icing, so prepare to leave it attached to the waxed paper - I cut a circle out before I started. Pipe royal icing on the circle, put the duck on, and then decorate around him. Small dots of white royal icing (with extra water added) look just like bubbles.
I used toothpicks and royal icing (no extra water here) to make the letters for happy birthday, and then just inserted them into the cake when dry. Has a nice 3-d effect. I used what was left of this icing to make little dots (like Hershey's kisses) of icing to decorate the other sides of the tiers. I put the leftovers in a little bowl for the older kids to enjoy.
I used a mix of cream cheese, cheesecake pudding and coolwhip for filling in each layer. I didn't have ANY leftovers.