My dog's job is to make sure I take my daily walks, and get out of the house (I work at home).
Otherwise, her job is to "supervise" me at work.
Fetch is a great job, and the hide-and-seek game. If you need more advanced mental exercise, you can teach her the names of various toys, and then have her go get a specific toy. You could work up to having her get things like slippers and the paper, too. Our last dog was great at that - Denna's still working on plain ol' fetch.
I'm working Denna in obedience classes now so we can get her Good Canine Citizen certification. After that, if we're up for it, we might look into Animal Assisted Therapy. There are some neat programs around here that use dogs to help kids learn how to read. I think that would be rewarding for both of us - but that also would be a lot of work for both of us.