High school: A very nice dinner with the whole family and my boyfriend.
College: Well, they paid for my wedding just 2 months after graduation, so I'll count that

(not the same guy as HS boyfried!)
PhD: A beautiful pair of pearl drop earrings that I still wear frequently.
For our kids:
High school: #1 got a weekend trip to NY; #2 went to Tiffany to pick out a piece of jewelry (we gave her a limit!), and #3 hasn't gotten anything yet. He graduated last year, and he never could decide what he wanted despite our (my) pestering. I really need to get him something. Maybe a Tag, haha.
College: Only one has graduated from college. She moved to NYC right after graduation for a job. We set up her new apartment for her - first mont's rent/deposit (maybe it was first + last, I can't remember), furniture, kitchen things. Nothing fancy but decent quality and comfortable.
Her sister has already decided that she wants to go on a trip to Yellowstone and Glacier when she graduates from college. And the youngest? Who knows what he'll want.
We try to tailor the present to the wants/needs of the individual kid, keeping the amount somewhere in the same ballpark.