When my son's best friend decided to have a Lego party for his sixth birthday, I started designing a Lego cake for it as soon as I heard. My family even sat with me in support and didn't call the psych ward as I sketched my ideas on a place mat at a restaurant at breakfast one morning.
For the cake, I made two 9x13 sheet cakes, one chocolate and one vanilla, just because. I used the part I removed when I evened off the top, placed into a cookie sheet and covered with tons of green frosting to even it out, as the base.
Then I created bricks by cutting the cakes into rectangles and pasting two rectangles together with frosting. (Note: I used more frosting than I have ever imagined on this cake). I placed the bricks in a pyramid pattern and then, after frosting first in white and then in color, I added mini Oreos and frosted them as the little interlocking thingies that make a Lego a Lego.
After all the frosting was accomplished I added several small ($3-$5) Lego sets that my son had a blast putting together. There was a street sweeper, a quad racer, a couple of random parts, and a fireman. I also, at our local Lego store, bought a bucket of individual bricks, including several staircases, flowers, bushes, etc., and with these I changed my pyramid into a Lego low rise apartment building. I placed the fireman on the steps leading to the top, where he was heading to put out the candles.
This was an incredibly fun cake to make. I even managed to get it to the party on time, in the rain, without dropping it or having it melt, and the birthday boy, family, and guests all loved it.