systemd-run is used as alternative to cron and at.
On Xubuntu there is notify-send to show on-screen notifications. It's required that the DISPLAY variable is set and it is sent as the correct user. Unfortunately systemd-run needs to be executed by root (I didn't find a non-root solution, yet).
# sudo systemd-run -E DISPLAY=:0.0 --uid manu --on-active="1m" notify-send -t 60000 "systemd.timer" "test" --icon=dialog-information