Dormont Pool is the best. I'm a non-resident so it's $7 to get in, but still an outstanding deal!
I always make sure to get there right at noon when they open to get a parking spot (the lot is small considering the size of the pool) and a good place on the grass. There are a few Adirondack chairs scattered around that you can claim on a first-come, first-serve basis.
They usually have music playing and there is free Wi-Fi, which is always a plus in my book. The pool is humongous so it never feels crowded, even on the most beautiful summer day.
I can't wait to go back!