Try more "basic" distros such as Arch, Slackware, Gentoo. Get familiar with the commandline. Some bash resources were already posted. Let me add my favourites:
Get familiar with the commandline package manager of your chosen distro. Then learn how to compile and make your own packages. Then how to add patches.
Play around with setting up services, like MPD streaming music, or an FTP or webserver. Look into firewalls (iptables) and other security measures. There is a lot you can explore.