A hand written letter telling them how much they mean to you is the first thing I would do.
I don't know if there are any keepsake items that your parents held on to since you were little. When i was in kindergarten in class we did our hand prints, a poem, and our picture in an 11x14 frame, when i graduated high school i did the same thing for my parents with a new poem about how far i've come because of them. They both hang side by side now on their living room wall.
Did you maybe do a silohette when you were little? In my area that was pretty popular in pre-school and kindergarten. you could do one of you with your cap on.
I'm pretty sentimental though and have already started little things with my kids that i hope to continue through out the next 17-18years.
As a parent, I can tell you what would mean the most is a note or something that says you recognize everything they have done for and truly appreciate it. That would mean 1000 times more to a parent than anything you could buy 
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