We used a bunt cake pan to make the volcano. Some candies chocolate rocks (which the kids loved most) and a bit of green colored shredded coconut for depth. I am not a cake decorator (as you can see) so I just winged it with canned frosting and colors.
To get the multi-color water river I literally dropped more green food coloring into the already filled pastry bag. I did not want to spend more money on a “sprayer” or other extra tools. Lots of fun. Good luck