One of the most powerful features of Apps Script is the HTML service. This allows you to create html/JavaScript applications which can be served up as webapps, run in sidebars or as add-ons and created great UI experiences. HTML service runs on your client PC, but it is also able to make calls and pass data to server side Google Apps Script functions.
Here’s a selection of things on this site that use HTML service
For more snippets like this see Google Apps Scripts snippets