Switch it, change it.
Luckily, for the sake of manicures everywhere, there's BrickStix decals. The idea for a removable type of toy-decoration was prompted by a 9-year-old boy who got tired of scraping stickers off his building blocks. Together with his family, he set out to invent a reusable cling, which affixes to most smooth plastic bricks, including Legos, Super Blox and Mega Bloks. With themed packs like Transport, Rescue or Pets, little builders can add a watchdog to their city, stick an alien aboard their spaceship or put a dent in their car. What happens when they decide their double-sided bulldozer is really a rocket to the moon? No problem. Just remove the current and re-stick another decal while changing the plot to the adventure along the way.