Depending on your requirements, the simplest way would be to use the popup notification applet. To show a notification use the 'notify-send' command. Here's a very simply script I run to show the uptime every 5 mins:
Code:
#!/bin/bash
x=1
delay="$1"
while [ $x -eq 1 ] ; do
notify-send -t 10000 -i ~/Azenis2.png "Uptime" "$(uptime)"
sleep $1s
done
exit 0
If you want to have user interaction, you could try zenity, it should be installed by default in Ubuntu.
for help.