I thought this might be a cool thread to brainstorm challenge ideas.
Here are some that I thought of:

- Dollar store challenge - craft a mer accessory using only materials and objects found in a dollar store/99cent store
- Design a game that mers can play (it can be as detailed as Quidditch, or can be an existing game that has been adapted). Bonus points for a video that shows you playing it.
- Photo scavenger hunt: name an object and do a photo shoot with said object incorporated (like I spy).