There are many things you could do. Here are some random ideas, but you can think of more:
Select a word plus click a button does a Google search, or lookup of the word in the dictionary.
Click a dropdown to select among n different fonts/styles for the page.
Use JS plus cookies to remember preferred font/style.
See notes on
Keeping state.
Turn blocks of HTML (like a section of the page)
into JS string constants using
backquotes (research it).
Then each time the page loads it can randomly reshuffle the order of page sections.
Write JS to detect mouse movements and (say) change colour/style of entire page as mouse moves about.
These are only suggestions. You can think of more.
A suggestion is to search for great demos of things you can do in JS and implement some.
Try to keep most JS in a separate file or files.
You can use JS libraries. But explain what is happening in your document.
Aim high:
Show me something achieved in JS
that I never knew could be done in JS.
Document
Submit a short document (3 pages or under) with it to explain it.
The document will take the major JS functionality and explain how the JS does it.
I want details in the document. Not this:
The JS does a search of the selected word
But rather this:
When button clicked, the JS function myfn() is called. This uses Window.getSelection() to get the selected text, and then ..
Include snippets of the important code in the document.
No need to submit all the code.
It is client-side, so I can "view source" to see the code.