A scale replica of the Taj Mahal.
An endless pit to kick your enemies into.
The cover of Queen's
News of the World album.
A house purely made of cacti, flowing lava, and explosives.
A mob zoo.
A Zerg Extractor to get more Vespene.
A single flower placed in the center of the empty lot.
Or you can go classic with a giant phallus made of solid gold; it's your choice.
Kitty fountains are also nice.