Start with a sandwich bag filled with random Legos, pick a theme ("Defend your town from a T-Rex" in this case), and then force yourself to use every brick in the bag.
Tell your story at the end.
A fun game to play when your backyard is like a swamp...
Final results:
My chicken powered mech with control station advance scout.
Katharine's command center controlled by the crazy sensei who likes to take naps and change disguises.
Lukas's assault vehicle and town barricade.
Lennon's triple cannon turret and barrier.
Keira's had a lot to say about this one, but I couldn't really understand any of it :)
Kassidy's solo cannon.
The T-Rex is in big trouble!