Python + Tkinter
python reminder.py
Works (or at least should work) with both Python 2.x and 3.x - tested with 2.7.12 and 3.5.2. Although more recommended is Python 3.x, as it is more stable when speaking of threads.
Also you need Tkinter to run the GUI. Check installation guide: http://www.tkdocs.com/tutorial/install.html
Should work on any platform (Windows, Linux, MacOS), but has been tested only on the first one.
Add recurring / routine tasks you want to be reminded about - e.g. "Drink some water!" or "Stand up and exercise a little bit!" or "Take your medicine!" or "Roll your eyes!" (no, seriously! Check this out: http://www.wikihow.com/Exercise-Your-Eyes)... You can imagine.
Each task will have its own thread, which will popup a message with a reminder.
Task interval also in minutes
Deamon functionality - start up with the system
Saving tasks
Task statistics, graphs etc.
Executable files
Better app architecture (instead of just one file)
Some tests...