I'm sure most here have played that online 20 questions game, which used user input to create an impressive database that would guess your item every time. What about using that kind of information as a captcha? Tell the user to do the answering for a predetermined item randomly chosen, and the user will navigate to the correct item via the answers. For instance, the captcha program says "Think of a %s ('bee', for this example) and answer the following questions:
CAPTCHA: Is it alive?
User: Yes
CAPTCHA: Is it bigger than a shoebox?
User: No
CAPTCHA: Does it fly
User: Yes
etc.
Sure, it would take a bit of time on the user's part, but it's fairly entertaining, easy for the end user, time-intensive on a large scale for scammers, and difficult for a computer to answer.