My kids birthdays are only 10 days apart so we have a joint birthday party. This year's theme was a luau. All my 9 year old son would tell me was that he wanted a "cool" cake, my 3 year old daughter's was easy, she wanted her favorite "upside down" cake (which fit perfectly with the theme). I decided on the volcano cake for my son, and it was super easy. I made a chocolate cake in a bundt pan, covered it in chocolate icing, dripped red icing on the top and let it run down the sides, colored shredded coconut for the grass, and used green Twizzlers for the trees (which didn't work real great), got some dry ice from a local ice cream place and we were ready to go. The kids loved their cakes!
You can make this cake to actually erupt down the sides but the steam effect was easier.