The Air/Space musem is great, but the wine valley traffic may be a bit of a challenge. I would definitely agree about going out to the gorge to see the various waterfalls, especially Multnomah. That is all on the airport side of town, so you might want to schedule accordingly. And in the other direction, of course there's the beach. But Portland itself has lots to offer in terms of restaurants, museums, gardens etc. There is plenty to do, and 1.5 days will hardly scratch the surface.. Have a great time.