An archetypical coming-of-age ceremony and rituals include dealing with a huge challenge and overcoming it. Frequently, challenges with the possibility of failure. Doing something like that in a secular setting isn't easy. After all, you don't have any tradition to advocate why the soon-to-be-adult should accept your hard challenge.
When it comes to hard challenges that actually make sense in our society, one of the rationalist complaints is that a lot of children chose a path of going to university without really investigating what the resulting career paths actually look like.
One idea would be to have the soon-to-be-adult cold call / cold message a bunch of people in the career paths that they think of taking to gather information about the career path and then report on the results of those queries at the coming-of-age ceremony to the gathered audience.