Sleep for 2 hours and 45 minutes:
sleep 2h 45m
Poor man's Pomodoro timer:
sleep 25m ; play bell.wav
Better one with tic toc:
(play tic.wav repeat 5000 trim 0 25:00 && play bell.wav)
Send a desktop notification from the command line:
notify-send "the message you want to show"
Pizza notifier:
sleep 15m ; notify-send "Pick up the pizza"
10...