It's really a matter of trial and error. Start with the fruits and/or veggies that are high in Vitamin A (meaty orange,leafy greens etc) and go from there.
Its really dependent on the bird. The ONLY fruits I can get Skittles to eat are grapes, apples and strawberries. Whereas with vegetables he'll only eat broccoli and sweet potatoes. So it's a process of elimination.
I'd advise against leaving 'treats' in dishes. ESPECIALLY fresh fruit/vegetables because it doesn't take long for them to get bacteria on them once exposed.
What I do for Skittles is if I am eating something that he can have, I share with him. Other than that, I feed him the Harrisons power treats. I don't leave them in a dish though. He 'earns' them through good behavior and he has to come get it.