You can make something that looks like a task list in a Leanpub book by using inline images, i.e. images that are part of sentences.
First, you'll want to create an image of a box, like this:
Then, you can write a task list in your manuscript, like this:
Here is a task list:
 Task One
 Task Two
 Task Three
 Task Four
Here's how that looks in a PDF:
You may need to adjust with the size of the box image and the size of your font until you get this right. Please note that it will look different in different formats, so what it looks like in EPUB may not be the same as the PDF, for example.
Typically, when people print ebooks, they print the PDF version. A PDF is also used for [creating print versions of your book](http://help.leanpub.com/en/articles/3333903-walkthrough-creating-a-print-ready-pdf-export-from-a-leanpub-book-manuscript-to-use-with-print-on-demand-services) for print-on-demand services.