For most graduates, there are no real keepsakes they will appreciate unfortunately. They will receive photo frames, albums, books (definitely don't get Dr. Seuss, it is so overdone), and random "graduation" themed items that they will leave behind at Mom & Dad's house!
The most appreciated gift will be cash -- they will have so many expenses that they aren't expecting, including the shock of school book prices, supplies, food, gas, etc. I graduated high school when I was 17 and received lots of cash which I responsibly used for things I truly needed because I moved out right after high school 3hrs away to attend a community college and had my own apartment. I had to buy all of the basic household items on top of pay for books & school supplies.
If you are set on giving them each a gift, I can only think of a few suggestions. A Keurig would probably go over really well with them because they can use it for coffee, tea, or hot cocoa! I would have loved one when I graduated since I was going to school & working at the same time.

I think a collection of 5 $20 gift cards to restaurants and/or a movie theater or a coffee shop (Starbucks?) would be great also because when they feel like they don't have any money to spend, they can pull out a gift card and have some fun. I still love receiving gift cards.