I remember Spreadsheet ML, an older format compatible with Excel. It had a subset of features, I think, but it was a rather powerful subset: formatting, formulae, multiple sheets. And it was rather simple. (Had a silly design mistake: for some reason MS gave namespace to attributes, which is not necessary, only for rather specific purposes).

Another XML standard from MS that also seems relatively simple is XPS, a PDF alternative. But it uses Open Packaging and that is somewhat hard to read.