Although I don't get there much, I love Waterfront Park. I've lived in a few river cities (St. Louis, New Orleans), and for me, Louisville has the best outdoor space along a river.
It's easily accessible -- you can have brunch at Garage Bar and stroll down to the park. There's lots walking (running/biking) paths. I love the Great Lawn and those bird statues (currently on vacation while they're restored). I love the Lincoln Statue, the swinging benches, and I really love biking through the park, always with the river in view. I love going to concerts at Waterfront Park and watching the sun set over the Ohio.
When the Big Four Bridge opened, I had a whole new reason to get excited about Waterfront Park. I love that looping approach. I always want to see more when we drive over bridges, and walking the Big Four lets me take my time and see all the river I want. I haven't been since the Indiana side opened, but I'm looking forward to exploring.
The Charles Rivers is also lovely, and I'm really excited about living close to an ocean. But I will miss Waterfront Park, and I will have to go to all the 2015 Watefront Wednesdays to make up for our time away.