I need ideas...

For something really unique, have you thought about Naoshima? It is an island populated by numerous art installations capped with a James Bond villain style Museum, Bennesse Art Site.

We have a forum posts on Naoshima already, so you should check them out if you are still using a JR Pass.

It's a strange thing, but many of the fun and unique experiences I've had in Japan are as much down to meeting interesting people as it is the actual place. You could immerse yourself in a side of Japanese culture you like (either in Tokyo or elsewhere) for the remaining few days and see what happens!

Hope this helps!