In my personal experience, I've always found that if I go to the park in the morning until mid afternoon, and then head over to Soak City at that point for a few hours, then back to the park for the rest of the evening, it is very refreshing. It often gets really hot while waiting in ride lines, etc., and a visit to Soak City mid afternoon is just what I need to keep me going. Now of course, this is very dependent on the weather, so adjust accordingly! Don't forget that Soak City also has Starlight admission starting at either 4 or 5 each afternoon, which can save you a bit of money of you're not getting a combo pass, which would give you access to both parks on the same day.