For most people ‘stop drinking sugary drinks ever’ would probably make the biggest life change.
And ‘the athletes plate’ would be the runner up bit of advice if you want something simple - half th plate veggies, 1/4 complex carbs, 1/4 unprocessed meat.
If you want to do it with complexity, count your macros.
Basically like you said, telling someone to not drink sugary drinks, stop eating out as much as possible and be more active is the only general advice really needed