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, such as giac.
Once you’ve started TeXmacs, you can interactively run giac within TeXmacs with the menu command Insert▸Session▸Giac. Once started, you can enter giac commands as you would in the command-line interface. The TeXmacs interface will also have a menu specifically for giac commands.
Within TeXmacs, you can combine giac commands and output with ordinary text. To enter normal text within a giac session, use the menu item Focus▸Insert Text Field Above. You can reenter a giac entry line by clicking on it with a mouse.