Crazy similes I have heard

• "As subtle as a train-smash"
• "As subtle as a sledgehammer"
• "As subtle as a huntsman spider in your undies"
• "Faster than an eel in a snakepit"
• "As busy as a one-armed carpenter in Smackaroo"

"You made that one up, didn't you?" Clancy looked embarrassed. "I did not!"
"You did! There's no such place as Smackaroo! I looked it up on the map! Anyway, if he's a one-armed carpenter, he wouldn't be busy, would he? He'd probably go fishing or something!" (The Last Continent, Terry Pratchett)

• "Nosier than a bandicoot on a worm farm"
• "As hungry as a hippo"

Feel free to add your own!