I second Bear Creek Lake park as it's a rather clean lake. I prefer the golden WW park for roll practice though. Once you get your roll going, practicing there will improve your skills more rapidly. Having said that, I go to Bear Creek Lake to get paddle reps and rolling in for just exercise.
On the river, I can abandon who I am and what I've done. However brief it lasts, while on the river I am nothing important and everything insignificant. I am flotsam, and happy to be so.