Dear {Supervisor},
Thank you so much for the opportunity to intern at {company}.
I've learned so much in the past {number of} {weeks/months}, and I'm now more certain about pursuing a career in {field}. Thank you for your constant {support/guidance} and for always encouraging me to do my best.
I'd love to stay in touch as I {finish school/complete my degree program}. You can reach me at {email address} or {phone number}. I hope there will be openings in your department after I graduate.
Thanks again for this incredible experience!
Best regards,
{Your Name}