Sitecore 9.1 is a pretty major release, and I've got a few topics I'll post on in the near future. But, thanks to the hard work of mostly Nick Wesselman, Habitat has already been updated for Sitecore 9.1. Simply clone the repository and follow the wiki to install. Likewise, J.F. Larente and his team have updated the modern Habitat Home site for 9.1, too. Habitat Home is a great demo site and also demonstrates SXA 1.8, so head that way for those instructions.
Even if you're not planning to use Habitat to peel through the Helix principles, or demo a site based on Habitat Home, the Habitats are great way to get content up on an instance of Sitecore to test what's new. Looking to install ML server and see how Coretx works? Why not do it with a site full of content. Need to test integrating with Sitecore Identity? This is also a good way to get started with that.
More to come as time allows.