Notepadd++ is good as well, as TheXero stated. Also if you use Linux gedit is an option as well as it will do essentially what notepadd++ will provide. Eclipse with Pydev can be used in Windows or Linux and overall I think its pretty solid. Everyone has their own tastes, just do what you feel most comfortable with.
About writing small GUI apps for python heres what I think is a good little resource:
http://www.java2s.com/Tutorial/Python/CatalogPython.htmCheck out the Tkinter and WxPython sections for GUI.
There are lots of resources you will find online for python, such as youtube videos, but I think this one is just good for writing a small GUI app for python and giving a quick example for an idea and you can just run with it.