One task that would be useful to promote to readers is adding links. This could be targeted at unlinked or underlinked articles, but it is one of the newbie exercises in EN:Template:Welcome_training, and it is a classic entry level task - no referencing required!
Topic on Talk:Reading/Readers contributions via Android
We're actually looking into this(!). How would you suggest we show the task to a user? Simply send them to a page that has a low link<word ratio? This phabricator ticket shows some early attempts to validate whether or not potential links algorithmically identified would be good canidates to suggest to readers for linking- https://phabricator.wikimedia.org/T148003 This was done at the link level, rather than the article level and my early exploration suggest "no", but if you have another idea or even speculation about how we could identify potential links or places that need links to a user, this could be a great first step to bring to more people and seems to already fit in our funnel.
In the welcome training template I used en:Category:All_dead-end_pages but you could do this programmatically and suggest articles that have been patrolled but have zero or few links.
In English the obvious things to link would include words that begin in capitals, especially if they match to other articles. The trick is to have something in the app that lets people choose whether to link to Dallas, Texas or Dallas, Scotland.
Another way to look at it would be to look at linking to newly created articles. I'd wait until the article has been patrolled, as otherwise most get deleted. But hundreds of articles are started each day and on average there will be several articles they could be linked to. You could also remove an orphan template if you succeed.