My sister is turing 30 and my BIL wants me to make her a exploding or surprise cake. By surprise I mean having the cake "jump out" or mash into her face when she goes to blow out the candles (sorry, but this is the only way I know how to describe it). I obviously don't want anyone getting hurt, but does anyone have any ideas for this? I have some time to work on it since her birthday is next March, but I would rather be prepared now or scratch the idea for something else. THANKS!!!

I read this somewhere, but have never tried it:
Do a layer cake, bake, cool and stack the layers, hollow out the inside with a knife and spoons. Air up a balloon with a can of spray whipped cream and fill the cake with the balloon. When she cuts into it the cake will explode.

If you want it to actually feed people, maybe this could be the top tier and the ones below could actually be sliced...