TeXmacs (http://www.texmacs.org) is a sophisticated word processor with special mathematical features. As well as being designed to nicely typeset mathematics, it can be used as a frontend for various mathematics programs, including giac.
Once you have started TeXmacs, you can interactively run giac within it by using the menu command Insert ▸ Session ▸ Giac. Once started, you can enter giac commands as you would in the command-line interface. You can later re-enter a giac entry line by choosing it with your arrow keys or clicking on it with a mouse. The TeXmacs interface also has a menu containing giac commands.
Note that a giac session in TeXmacs may be started directly from a terminal using the script xgiac available in the src subdirectory of the giac source.
Within TeXmacs, you can combine giac commands and their output with ordinary text. To enter normal text within a giac session, use the menu item Focus ▸ Insert Text Field Above.
You can also use mathematical input when entering commands, which allows for fast typing of complex formulas. For details, see the giac plugin documentation in TeXmacs. Also, any command output in a TeXmacs session (or a part of it) may be copied back to an input field, thanks to the conversion routines between giac syntax and TeXmacs Scheme.
Graphics created by giac are converted to PDF and embedded into TeXmacs sessions. Note that this requires the Ghostscript package to be installed on your system.