A few ideas to spark interest -
- Photos of themselves: humans being what they are, mostly more interested in themselves than anyone else - perhaps in poster form, present 3 to 5 photos of everyone (a young one, a current one, and others in their prime, or representative of their personality), with the idea of letting them choose which one they like best for their primary WikiTree photo. Most people can't resist something like this, may also offer a different photo. Then lead them to photos of ancestors and ask which is the best one for them, hoping that someone will recall a better photo they have and get it for you.
- Stories: most people like good stories, both about themselves and about their relatives, so present some of the stories you have found that they may not have heard, and ask for more. You can start by asking for the funniest and weirdest and most bazaar stories of themselves and ancestors, but slip a question in about what would be a story for each ancestor that best represented who that person was, a story that they would most want to be remembered for, always with the idea that you are looking for content for their bio's on WikiTree. That should whet their interest in WikiTree, and what you're writing about them and their relatives!
- An idea for later, an informal email with a story and/or unusual fact from their ancestry. I've wanted to create a roughly monthly email that mentions one or more of the most interesting thing(s) I have found out recently. Now and then, you uncover a really interesting story, sometimes funny, sometimes tragic, sometimes heroic, and you know that hardly anyone has even heard of the person. This is a way to introduce an interesting ancestor, with their story. Of course, you would always refer to their WikiTree page!