Clean an installed Fedora OS:

clean /var/cache

/var/cache/Packagekit

Source

sudo pkcon refresh force -c -1

rm -f /var/cache/PackageKit/metadata/updates/packages/*

Disable the saving of the rpm's:
Edit /etc/PackageKit/PackageKit.conf and remove the # of KeepCache

# Keep the packages after they have been downloaded
KeepCache=false

dnf

dnf clean all --enablerepo=\*

clean /var/log/journal

Source

You can delete everything inside of /var/log/journal/* but do not delete the directory itself.

You can also query journalctl to find out how much disk space it's consuming:

$ journalctl --disk-usage
Journals take up 3.8G on disk.

You can control the size of this directory using this parameter in your /etc/systemd/journald.conf:
SystemMaxUse=50M

You can force a log rotation:

$ sudo systemctl kill --kill-who=main --signal=SIGUSR2 systemd-journald.service

You might need to restart the logging service to force a log rotation, if the above signaling method does not do it.
You can restart the service like so:

$ sudo systemctl restart systemd-journald.service