Spike's favorites are his everlasting treat ball, and an empty water bottle (like a 500 mL bottle of Evian), with some kibble in it. He bats the bottle around to get the kibble out, and it keeps him busy for hours. It took him a while to get the hang of it, and now he's obsessed.
He also like his kong. I put some kibble in it, then plug the hole with a bit of wet food and freeze it. That usually keeps him busy until he gets all the kibble out!
I have links to some great toys on my website, including the Nina Ottosson ones.
Everything For Your Yorkie