Here's a little project I had to make a 1940s style faux cake for an exhibition.
Lift the cake and inside would be a piece of fruit cake, which is what they did in the old days as they couldn't afford anything glamorous.
I first made the main basic shape of the cake with plastazote then covered it with lightweight filler. Once this was dry I sanded down the surface and painted the cake with white/cream colour acrylic paint with a mixture of PVA glue. For the cream decorations on the cake I used the lightweight filler again using the piping bag, just like decorating a real cake. I got some bits and pieces of decorative items such as fake flowers and ribbons from a haberdashery store and arranged these on the cake. I used a glue gun to stick these onto the cake and it was finished! Very easy and quick. Lightweight filler was great to use, the cake doesn't weight a thing! :)