All I had for Max was one of those ball throwing sticks,we'd sit at the top of the hill on the old pit tip and he'd chase his ball up and down all day long.
Sainsburys has some quite good wild ducks that the spaniel here adores, spends hours wandering around with it in his mouth....but then he does that with his dinner bowl too!
Dunno about stimulating as such, but my dog adores her donkey teddy bear and carries it everywhere. I've lost count ive woken up with her and the damn thing in bed with us. As I type, she's next to me on the sofa, using it as a pillow.