I would suggest something special that they will keep forever. Either diamond earrings, or a charm bracelet that you can put about 5 or more charms on each. You add things that remind you of them. They have EVERYTHING! I started one for our daughter on her 1st bday...so far she has the intial "A" for her name, a bday cake, and a ladybug. I call her ladybug as her nickname. She is 3 yrs old so by the time she is 18, she will have 18 charms on there of my thoughts of her over the years. They have sports balls, dancing shoes, every single thing you can think of! Go to a jewelry store and there should be a book you can choose from. Literally anything you can think of will be in there. If they play piano, if they play sports, if they love flowers, if they love a certain car (I have a volkswagen bug on mine b/c it is my fav car), if they want to be a nurse, if they love purses, shoes, shopping, etc
For my wedding I gave my husband a watch engraved on the back. You could give them a really nice watch and have it engraved with a special phrase, word, reminder of them, or just the date. A watch that will last a life time.
My dad bought me a heart shaped charm necklace with diamonds all around it for college grad. I didn't like it or appreciate it until my 30's though. My mom bought me diamond earrings. For high school I got a new tv and things to set up my new dorm room at college.
A friend had a scrapbook made for her daughter. She gave the woman tons of pics from baby to 18 and the woman scrapped them. She paid A LOT for it but her daughter LOVED it!
You could do a down payment on a car. You could buy them a really nice IPOD. Pay a few months in advance on a new apt or furnish it for them.
You could send them on a weekend trip. My dad and his wife also took me on a skiing trip with them and that is where they gave me my necklace. Take them to the beach or skiing...maybe even let them bring a friend and get their own room. Or a 3 day cruise if you can afford it.
How fun! W.