Start your favourite game when you want to give yourself a break.
Alarm Clock has loads of options you can use for each event. Fire it off on certain days and months. Play a sound, run a program, show a popup or even lock the screen. Create templates for common tasks.
Need more time? Use the snooze button to get the alert again a few minutes later
Be sure to spend time looking at all the options because there are a number of menus that each have useful settings for customizing how you use the program.
Finally, there is also an applet version that works in a similar way but is more concise. It's up to you which one you use, though I prefer the main version.