Choc. Choc and more Choc!!!! One of the easiest cakes I did that ended up looking neat was a choc. cake with choc. buttercream. I took choc candy melts and melted in the microwave then poured it in a pan lined with wax paper. After it set up take it out of the pan and drop it on the counter. Use the pieces to make a mosaic. You don't have to worry about smooth sides or square edges-nothing because it is all covered with choc bits 
you could make a stethascope for a cake topper easily out of fondant/gumpaste. I found it was like using playdough or plasticine.....if you are uneasy about doing it in fondant first, do it with playdough and see how it looks...you can find pics all over the web for them...
I think choc is a great idea too.
I have and idea that is kinda like NEWTODECORATING. . . you could use chcolate cake chocolate buttercream, but then you could just find like little different candies(chocolate) and put them all over the cake, you could also sprinkle chocolate shavings on the top or something.
Casey 
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