I have owned both D90 and D7000 and would tell you that the D7000 is a definite upgrade from many perspectives. That said, if you are doing what you need with the D90 there is no need to upgrade the camera. That is the logical answer.
However, I feel that my fellow Wildcat deserves a new toy and there are a couple of ways of going about this in a financially responsible manner. To that end I would second Will's recommendation of a couple of prime lenses. Think of the fun you could have with a nice 20mm Nikon lens shooting Deering Library from Sheridan road. By using a fixed focal length lens you will need to explore different angles of composition and your photography will improve. These primes are quite sharp and are built well. You could pick up a used copy for a reasonable price. If you change your mind or if the focal length is not to your liking, sell it and buy another. If, in the long term you decide to switch to FX, you already have some nice lenses to start out with. As I do a lot of hiking, I have switched back to light primes which take up little space and can be placed in your coat pocket. If you have no intention of moving to FX, the 30mm DX version is a great little lens.
Whatever you do professor, get a new toy for yourself. The grandchild will not appreciate it as much as you will.