What if you used a combination of the captcha ideas you had (choose any of them - or select one of them randomly):

• ASCII text captcha: //(_)//
• math puzzles: what is 7 minus 3 times 2?
• trivia questions: what tastes better, a toad or a popsicle?

with the addition of placing the exact same captcha in a css hidden section of the page - the honeypot idea. That way, you'd have one place where you'd expect the correct answer and another where the answer should be unchanged.