This was totally awesome, it worked great! I forgot to add the dish soap, but didn't realize it until I saw it sitting out on the counter a little later LOL. It was fine without it, I don't think it could have gotten any more bubbly :) Thanks for a great idea!
Used to make a simple version of this (just baking soda and vinegar) with the kids and when I was a kid! Your additions sound great. We used clay instead of dirt which is easy and looks nice. You can use brown for the "mountain" and have red clay coming down the opening like hot lava.