An extremely lightweight option may be to use HTML tables. Just create head, body, and table tags in a file, and save it as a file with an .xls extension. There are Microsoft specific attributes that you can use to style the output, including formulas.
I realize that you may not be coding this in a web application, but here is an exampleexample of the composition of an Excel file via an HTML table. This technique could be used if you were coding a console app, desktop app, or service.