Sunflower seed and seed in general has really gotten a bad reputation. It is NOT unhealthy for birds when it is only a PART of a balanced diet. The problem with seed come from people who feed their birds nothing but seed. It's kind of like how if you put some butter on your bread, it's not bad for you, but if you eat a stick of butter every day and little else, it would become bad for you.... Seed would make a good foraging treat since it does not spoil
Fresh produce on the other hand is not a treat, it's a dietary staple, and a MUST for healthy birds. You should offer fruits and vegetables every day (please familiarize yourself with what is safe for bird to eat

). A bird should get a wide variety of produce each day (many people make chop mixes to be able to offer a wide variety).
"Treats" are things that a bird likes, but probably shouldn't be a major part of their diet. For treats, I'll give my bird noodles, a small chunk of bread, a cracker, a piece of cereal, a piece of popcorn.... things that aren't outright toxic, but aren't something he should have every day or in large quantities either. I *personally* don't use treats for training, just as a way to be nice to my bird (and sometimes, shameless bribery lol). For actual training purposes, I prefer kisses and verbal praise. I don't always have a treat on me when my bird does something good or shows a new behavior/noise I like and want to reinforce. I can, however, always give him a kiss and tell him he's a good boy

and when his expectation is praise and a kiss instead of a treat, it makes life a lot easier.