When you get there, or before you leave, or whenever.. just find the right time... just tell them that you love them and appreciate everything they've done for you/with you, and beyond that, simply acknowledge what you like about them as just people in general. Bring out what's special in you as a reminder to them, to show what they've accomplished through raising you.
Oh, and definitely don't forget to be as cliche as possible! :D
BTW mix in some of your own creative ideas with the ones you believe would work great on here. You know them and yourself.. well, I assume so. Perhaps you can inject some additional unpredictability in there, Idk.
Good luck, but whatever you do, what really matters is your intention and what's in your heart. :)
Of course, the best gift.. is time.