Is this something that I should do, or will it be cheesy? Is a
thank-you email too impersonal? I don't even know what a thank-you
note consist of...do I go full on with a card and envelope, or
It won't be cheesy at all, and if sincere, will be remembered and appreciated.
Over the years, I hired many interns. On their final week, I took them out to lunch to talk about their experiences, ask them for suggestions on what I could do better for future interns, and to thank them for their hard work. Usually, they thanked me in person as well.
Some of the interns sent a follow-up letter thanking me for the opportunity, mentioning how much they had learned, and hinting that they would be amenable to a full-time job once they graduated. On several of those occasions, I took the up on the offer and did hire them when they became available.
The specific format wasn't very important - sometimes it was a thank you card, other times a handwritten letter, sometimes just an email. Far more important was the contents. A sincere thank you goes a long way.