Please let me first assure you that I'm speaking from experience here. Your post is pure music to my ears.
I have a son who has ADHD and ODD (Oppositional Defiant Disorder.) I had heard of ODD, but thought it was just an excuse for poor parenting, then my son started to show signs of it. My child's therapist (he started in therapy at 12, and we started in family therapy) showed me the criteria in the DSM-IV; my son met every single criteria!
Though I was also in therapy and spoke to his psychiatrist at least monthly getting coaching on how to deal with him, my son still managed to make my life (and probably his own) miserable. He was secretly getting drunk, was cheating, stealing, and failing high school, picked fights with me and his brother constantly (though we would not engage), sent his brother to the hospital with a concussion, and I became very depressed. I also thought I was the worst mother in the world. My husband refused to believe there was anything wrong with him even though the psychiatrist was telling him otherwise, and ended up leaving me. When his principal called me a few months later to tell me that they were going to kick him out of school, I had an emergency meeting with his psychiatrist. I ended up sending him to the wilderness and then a therapeutic boarding school for two years.
As I said, my husband didn't believe there was anything wrong with him, and wouldn't pay for any of it. The therapeutic boarding school cost me my entire retirement account and a second mortgage.
Yet here comes the good news.
After a lot of struggling and therapy, my son graduated from the boarding school with new life skills and got an academic scholarship to college, which he kept for four years. He graduated magna cum laude, went to medical school, and is now a respected doctor, has been married for several years and is expecting his first child. He turned his life around completely. And that is all the thanks I need. I'm dead serious. His life now is reward enough. He, too, has expressed sorrow, and we have a good relationship.
My guess is that your parents feel the same. You sound like you're a wonderful son, and they must be so proud of you and the way you turned your life around. Your new life and the fact that you make an effort to spend time with them and thank them for sticking with you is most likely all the thanks they will ever need; save the gifts. You have nothing more to prove.
You feel guilty about what you put them through (which is understandable), but they have forgiven you (they have a loving relationship with you; that's the proof.) It's time now to start forgiving yourself. Until you do that, you will never feel you're doing enough.
Read some books and articles online about guilt and forgiveness. If you need to be concrete, show them your post. Ask your parents formally if they could forgive you for all the difficulties you caused them and get it verbally. Ask if there is anything they want you to do that will show them how sorry you are and how grateful you are for their love and support. Then cut yourself a generous amount of slack and forgive the child/adolescent that you were (that's not who you are anymore), and the stupid, hurtful things all children do to some extent. You couldn't control your impulses then. That doesn't make you bad; it makes you flawed, which is the human condition.
Congratulations on your new life, and I hope you find self-forgiveness and freedom from guilt.