I've been trying to find some things for Thor to do by himself as well. He was not that interested in bones, so I tried soaking some flossies in water before I gave them to him. He went NUTS. He's very small, so I think the extra malleability helped his jaws. He has actually jumped off my lap and gone into another room to work on his flossie.
You could also try those treat puzzle balls to stimulate your boy's big brain.
Also, maybe don't freeze the kong? Just put some peanut butter in it and let him work on licking it all out.
Finally, maybe give him more intense outdoor exercise so he wants to sleep for a while when he gets back in. Fetch is good.
Oh, and one more final, final thing: since he is so smart, maybe you can work a flag into the bell training. Train him so that if the bell is covered by a piece of cloth, ringing it won't get him attention. Or just take the bell away for a bit. Presumably he's not going to need to go out again for at least an hour after coming inside.