I once saw a Trisha Gee presentation where she suggested having a pet project that you try re-implementing every time you learn a new framework, like a richer form of 'Hello World'. I've subsequently found out about https://github.com/gothinkster/realworld - this project is a microscopic version of that!
This is a monorepo that will contain a project to create a rough note-taking application (similar to Evernote, for example) using differnt UI frameworks. This is not intended as a production application.
First attempt will be via React, the UI framework I have most practical experience in.