There's an idea that I think is quite good, easy to implement and better than most unreadable CAPTCHAs : display 20 thumbnails of animals (or use your preferred category) randomly arranged in a grid, and simply ask "please click on the cat" (or the dog, the pigeon or whatever). It's cute, it's easy (some CAPTCHAs are seriously unreadable for real humans too).
If you think random clicks are too easy for bots, than ask "please click on the cat, then the dog, last the cow" and it should defeat any randomly clicking bot if necessary.