Add a single reminder to your menu bar - always visible.

You can enter any text that you'd like to have on the menu bar and it will be shown to you at all times.
You can also activate an hourly sound effect. This could e.g. be used to stand up or stretch once per hour. The possibilities are endless.

Use Dojo to actually do what you want to. Dojo reminds you to stand up, stretch or finish that assignment.