Here is the guide for Rails Girls http://guides.railsgirls.com
The app you build in the Rails Girls tutorial is essentially an instagram/twitter clone... a user can have an account/profile, add images and text and I think comments are functional as well.
My "scavenger hunt" app would implement all of these features but it would also need a feature like this:
A menu (such as a drop down) to select which item on the scavenger hunt this is, i.e. this is "take a picture of me at my job", which could be one of 20 options. Then, according to which item they selected either text or an image is optional or one or the other is required. So, for instance if it is "take a picture of me at my job" text is optional and an image is required. Also each item is worth a certain number of points. So, if the "take a picture of me at my job" item is worth 20 points and this person works three jobs then after posting three of these types of pictures they would have accrued 60 points and this information would be associated with their specific user account and would display along with their profile information on a profile page or next to their name and profile picture when they are posting.
Thank you and any help is very much appreciated.