A Biking Trip
He'll be back at school studying hard soon enough. So why not treat him (or her) to a biking trip? There are several companies which organize these action-packed trips. Check out Teen Treks and Overland, for example.
A scooter is sustainable transportation especially if she will be going to college in an urban area without much public transporation. The Honda Metropolitan, for example, reminds me of the classic Vespa (only less expensive). It's easy to operate and light on the pocketbook.
There are hundreds of makers and thousands of models of watches to choose from as you well know. In every price range too. From Michele to Casio.
The bespoke shops call them writing instruments. Whatever you call them, a fine pen is a gift to be cherished. Mont Blanc and Cross have a variety of models from which to choose.
Make your gift memorable or a keepsake or both. Best wishes to you and your family at this happy time.