Dear …
I want to thank you for speaking at our__(type of seminar)__ seminar on __(date)__. You did a commendable job. It was both enjoyable and informative. You captured the interest of your audience and your expertise came shining through with the delivery of the messages you conveyed.
We received very positive comments from those who attended the seminar. They really enjoyed hearing you speak and found the information to be quite valuable. Once again, thank you for your tremendous contribution to this event and your help in making it a great success. Your knowledge and your efforts are a credit to __(this community/our company/our firm/our organization/etc)__.
Sincerely,