One of the things I use a lot is Vertical Text Selection or Block Text Selection. This allows you select text vertical down multi-lines and make changes to all selected text at once. In the picture above I've selected everything in grey, "Multi-Line", when I type, all the "Multi-Line" text will change to whatever I type, "All Changed at once".
To use this feature just hold down the Ctrl (control) key and select the text with your mouse (hold down left mouse button and drag vertical over desired text). This is a very useful feature when using Geany for program code editing.
This isn't the only thing that Geany does but I had trouble figuring this out so I thought I'd share.