You don't say exactly how much time you are taking each way but you should try a different route in each direction. Of course the Grand Tetons and Mormon row to the south of Yellowstone are popular and for good reason, they are fantastic ! If you exited the park via the south you could visit Craters of the Moon, Shoshone Falls [although drier in the Fall still a great sight] and Perhaps stay in Twin Falls, home to the Snake river canyon and Perrine bridge where you can witness 'Base jumpers' throwing themselves off the top ! Then it would be back through Boise towards Seattle, again if you had time you could drive the Columbia river Gorge through to Portland.

Now if I had an extra couple of days heading out to Yellowstone I would certainly consider visiting Glacier NP and driving the Going to the sun road if indeed it was open but even if not, it's still a great place to visit.