Apps for (very!) simple spreadsheet purposes
Numbers provides customisation to its UI which allows you to strip most of the features from the main window, leaving you the most basic of functionality.


The toolbar is customisable (right-click on toolbar and choose Customise Toolbar). You can use this to remove features you don't wish to use so that they do not show on the toolbar. You can also go further by hiding the toolbar completely.
The inspector on the right can be hidden from the View menu (View → Inspector → Hide Inspector). All that's left is the main window with the spreadsheet.


Hiding the toolbar and inspector gives you a window that looks like the following. UI can't really look more basic than that!