My son asked for an army party with an army tank cake. I got some great ideas from this website, then added some of my own touches.
To get the tank shape, I baked a 9x13 cake, cut it in half, then put one half on top of the other. I baked some cake batter into a small, round, oven-safe dish to create the smaller section on top (I wish I had made this part smaller).
I covered the whole cake in chocolate frosting. Next, I put on the wheels. Initially, I was going to use Oreos, but when I was at the grocery store, I walked by the donettes...I think they look more like tank tires and I'm convinced they were what 'made the cake'. I simply dabbed some frosting onto the back of each donette, then put them in place.
The next part was not hard, but it took a long time. I used the Wilton star tip to do the camo look. I felt this was the best approach, because I didn't want the different colors to run together or look sloppy.
All I did for the coloring was use chocolate frosting, tinted with black coloring or white frosting to achieve different brown shades. I did the same thing for the green shades. To get olive green, mix chocolate and green frosting together.
I would suggest making the cake a day before the party. That way, you can take your time with the camo frosting. Work on it for 15 minutes, then take a break for an hour or so. Just keep working in little spurts until it's done.
Lastly, I made some marshmallow fondant. It's not as hard as I'd anticipated. I colored the fondant black, then rolled it out. I cut it into 4 strips. I took a strip and gently laid it across the top of the tires on one side. Then, I took another strip and slid it under the bottom of the tires (this was a bit tricky). Next, I trimmed excess length off either strip and sealed them together with my fingers. I repeated this for the other set of tires on the other side.
If I were to do this again, I would take the time to make tread patterns on the fondant to give the tank more character and detail. To top it off, I wrapped extra fondant around a pirouette stick and gently pushed it into the front of the top section of the cake. Crushed Oreos were placed around the tank to look like dirt.