For a baseball-themed party, I created Fenway Park. This was a LOT more work than I expected, so please read these tips!
The cake is a standard Devil's Food. I worked up a PowerPoint that enabled me to cut a single 9X12 cake into pieces that assemble into precisely the shape of Fenway Park. This creates three problems:
1) The cake needs to be flat on top for the pieces to line up. I don't like wasting cake, so I did not cut the top of the cake. This was a mistake and you can see the seams in the picture. If your cake is not pretty flat on top (even if the top will be the bottom), you should get a sharp knife and cut the top so it's level.
2) Small pieces of cake have little structural integrity and tend to sit on the board at whatever angle they please. This creates visible seams where pieces meet. Pick a moist cake (I did) and use toothpicks to hold the pieces together horizontally (I didn't). Also note that filling seams is easier with a crumb coat... see below.
3) Cut cake is "crumby." Whether you've cut the top or the sides, there will be crumbs that get into the icing. To fix this, bake the cake and, as soon as it's cooled to room temperature, cut it, toothpick the pieces in place, and ice it with a white icing. Don't worry about crumbs getting into the white "crumb coat" as no one will ever see it. Then put it in the fridge overnight. The crumb coat will harden and give you a strong base on which to decorate. (I've never used fondant. That may be better all around.)
The decoration was green home-made cream cheese icing. I then created a stencil by printing out and cutting out the base paths. Through the stencil I pressed graham cracker crumbs created in the blender. Next was the pitcher's mound. At scale, this was much smaller than expected and literally a mound of graham cracker crumbs with a tiny bit of white icing mixed in as a binder sufficed. I opted not to do a warning track as I ran out of time and, with the complexity of the outline of the field, it would have been a lot of work.
The base lines, coaches' boxes, bases, and on-deck circle are all white icing. The foul poles are yellow birthday candles. Note, in my fatigue, that I put Pesky's Pole in the wrong place!