For a Hawaiian themed party I was having I decided to make a volcano cake. I used 2 package mixes of chocolate cake in a bunt pan. It overflowed so I had some extra cake bits, I grounded those up and spread them all over the cookie sheet to look like dirt.
I used two already made icing containers. One chocolate that I smoothed all over and the second vanilla that I split up evenly into Ziploc bags and dyed with food coloring. I then snipped the edges and drizzled the icing in layers, even managing to spell out Happy Birthday. I then fit a glass in, poured hot water in and put a few cubes of dry ice for the smoke effect. It was a real hit.
The cost of the cake and icing was less than $8, the dry ice was 20$ for a 5 lb bag (that was the smallest they sold it in and was a huge amount -bought in Ontario). I baked it for about an hour and 20 minutes and it took an hour to decorate. It was really nice and moist, probably from the steam coming out all night.