A few years back I did something called 'master the mainframe' where they provided a TSO login for you on an instance running in one of their labs. They also provided you with a bunch of fun exercises to learn more about the system.
"The IBM Z Xplore Learning Platform (formerly called Master the Mainframe) is a fun way to get hands-on experience across a variety of technologies, to develop valuable skills, and to earn digital badges – with no prior knowledge required!"
I've had this bookmarked for years, never gotten around to starting it, though. Web development continues to entertain, challenge, and sustain me.
I think this is the equivalent - https://ibmzxplore.influitive.com/users/sign_in