If you aren't super pressed for time (this takes me about 45ish min I think), you can go out the GAP trail to McKeesport, get on the steel valley route, and then cross the sidewalk on the mansfield bridge, go through Dravosburg and down to the bottom of the abandoned Curry Hollow road and climb up. This will put you right next to the Holiday Inn by the airport. This my preferred route because there is little traffic. You might find it a little sketchy but I have never had a problem. Pavement is rough and washed out in places. Because the road is closed, google maps won't let me complete the route but from where this ends, you would just go between the barriers and up the hill.
http://tinyurl.com/p3gy8cs
I personally will not ride up 885 except on rare occasion but I do ride down. I find it terrifying to go up.
If you don't have as much time, t his ride is a good way from the waterfront to the airport but you will need to ride on a sidewalk with glass in place. I use gatorskins and don't have problem.
http://tinyurl.com/nm5d49h To go back to the waterfront, I would go down 885 by reversing the exact route posted by Marko.
Another fun route starts like Marko's instead of turning right at 885, go straight just a little bit on Mifflin Rd and climb up Mooney Rd on the left. It is also closed to traffic. Turn right on Lindenberg and right on Interboro then left on Mifflin Rd. Then right on Lebanon Church and you have to take the sidewalk of glass again to the airport.