I like the Kong Wobbler. It's not TOO difficult but still stimulating enough. You just have to push it, and food falls out of the hole.
We've used this one before (friends dogs we watch sometimes have this):
Amazon.com: Company of Animals Nina Ottosson Dog... and the problem is, Jackson likes to try and eat the plastic pieces after he eats his food, lol... He picks them up and starts chewing on them... but if your dogs dont usually eat plastic, it's good!
I really am for using these kinds of toys though (even for meals). It tires them out mentally!