-Like most people here said, money is always a good option especially for people who probably have a lot of loans at this point (if we're talking grad school).
-If you're looking to spend more money, you can gift them with a getaway package- a small weekend trip to celebrate (with you or with someone else)
could be fun (a big stuffed tooth for a new dentist or something like that!)
-Along the cute stuffed animal route, you could get a custom stuffed animal at www.budsies.com. They do stuffed toys based on artwork or photos (so it could even be a stuffed animal of the actual person dressed in the appropriate clothing for their new career!)
-Something engraved/personalized: jewelry, a watch, or a leather-bound set of journals (something academic/professional feeling)
-Depending on the chosen career, something related to that