My daughter’s 5th birthday party was at a local amusement park and she wanted a Roller Coaster Birthday Cake. Specifically, she wanted a 3-D dragon coaster which is a kiddie ride they have there. So I baked a 2-layer rectangular sheet cake with and then cut out a semicircular piece out of one side (really sort of parabolic more than semi-circle).
I froze the cake for about an hour first to make it easier to do this part. I then inverted that piece on the other end of the cake. I stuck a couple of drinking straws vertically through the hill to give it a little more structure. Then I frosted all around, and piped the track & added bushes for accents. She made the Lego dragon coaster and starting station, complete with the guy with the control lever.
It was really easy and came out looking great.