I didn't see it in the recipe book, but is there an easy way to add an "add to calendar" button to HTML templates for email confirmations in v16?
Not yet. But please go to the Ideas board and vote for it to be considered for a future release.
community.tessituranetwork.com/.../ideas
Specifically community.tessituranetwork.com/.../include-add-to-calendar-links-in-ticket-email-messages-and-tnew-order-confirmation-page