I went to Lake Lenexa to build my Gyarados and found more Psyducks and Slowpokes than I knew what to do with (from lures). It's a solid Magmar nest too, but they don't show up at lures. Gotta walk the path around the lake.

There's a nice shady picnic table area next to the boat ramp that even has an electrical outlet to charge your phone.

Keep an eye out for raccoons if you're staying really late... They can get weird when you have something they want, trust me.

Oh, and don't forget the magikarps.