The Stash block allows the teacher to configure each stash item with a graphic, and then it can appear as either a graphic, text link, or both (all customizable by the teacher along with "hints" that show up in the students Stash block that you configure in the Item Details). In the teacher report view you can see which items are in each student's stash, and you can all reset their stash back to zero.
You can configure different "Locations" for the stash items to show up (you get a code snippet to paste into a text editor box which the Stash filter parses out), and you can set the availability of the item (e.g. gold coin appears up to 5 times every 3 days, etc. to incentivize participation, or you could hide it in a feedback field as a reward, e.g. once you score above 90% on a quiz, you receive a golden coin!).
The Stash block looks like it has a lot of potential! Basically, it's kind of like the nifty built in Certificates and Badges in Moodle (which are also great for gamification!), but with more flexibility! You hide a "stash" object anywhere--inside a label, page, quiz question, quiz feedback (e.g. earn a 90% or more and it unlocks a "stash"), BUT you can set some special conditions that are totally unique compared to Certificates and Badges (e.g. the stash can only be accessed 7 times--only 7 gold coins!--and then only within a certain time window will the stash re-appear, e.g. after 30 minutes).
Looks like with the Stash Availability plugin you can also set conditional access within Moodle based on whether or not a student has a certain stash item in their collection (e.g. can't access the Zombie War Room unless you have the Zombie Access Code Skeleton Key!). How cool is that? :)
Great overview of using the built-in conditional access tools to gamify your course--using simple graphics and labels that are revealed as items are completed, quiz scores achieved, etc.
You can also hide "Easter Eggs" using the "Orphaned Activities" trick--put items in a section at the end of your course (e.g. section 10), then reduce the number of sections of your course (e.g. 9 sections). When a student achieves what you want them to, you can the put up a link / graphic to the "orphaned activity" in the section at the end of your course. This works because the link is made available to students, but not in a hidden section that restricts course availability!