What a sweet name! Post some pics of little Sushi!
As for things to do... I toss around some smaller tennis balls with Hiro and while I do that I teach him fetch/bring it back/drop it. I mostly just play the usual snatch and grab games with him too! lol I don't know anything really different to do.
I also take the time when he is playful to try and teach him some basic commands. Since he likes to make me happy this works out well for both of us!
I think that mental exercise is just as important as physical exercise but I know when Hiro gets tired of thinking and learning and when to stop lol
I find that the best way to tire Hiro out is to take him on his walks. This works physically and mentally I find... Lately I've been slacking since it's starting to get cold outside, but I've been paying for it because he's been naughty lately. lol It gets dark really early now and the husband doesnt like me to be walking in the dark... and since I don't have a yard, I have limited time. I suppose that you could even take Sushi out to a closed tennis court or a dog park (if you're ok with playing with bigger dogs) to let him tear around. We don't have any snow yet, but I'm looking forward to see how Hiro reacts to it!
Hope I've helped a little! Good luck!
Maybe I heard somewhere that Yorkies are good at agility?
Maybe if you have some doggie agility classes around that might be fun? I don't know anything about it, but I enjoy watching them!!