For a quick fix, check out Slippery Rock Creek and Neshannock Creek. Both are an hour north of the city. They aren't pristine mountain streams, but they'll keep you entertained. Stocked and hit pretty hard early in the season, but the pressure (and fishing) probably tapers off as the mercury rises. If you keep going past those two creeks, you'll find Oil Creek (where I've had some amazing days) and the Allegheny National Forest. ANF offers some awesome streams with very little pressure. I frequented the ANF when I first picked up a fly rod at the age of 10. My first fish on a fly was actually on Salmon Creek in the ANF. A brookie crushed a muddler minnow I was dragging through the current. I had no business catching that fish... I had no idea what I was doing. Remember it like it was yesterday.
If you want to stick close to home, the Allegheny and Ohio rivers offer some awesome bass fishing. A boat definitely improves your chances, but I'm sure there are decent places you can access from shore. I'm not sure about the Monongahela... might be alright too. The Bassmaster Classic was actually held in Pittsburgh back in 2005. So you know there's fish in there.
Looking forward to that new Orvis store that Dan mentioned. I was born and raised in the area and have grown to love it even more now that I don't live there. Best of luck!