What’s the easiest way of adding more options in the Theme Options of the twenty eleven theme?

As you can see in source theme makes some use of Settings API, but doesn’t include calls like do_settings_fileds() that would allow you to use add settings.

I would probably try to unhook twentyeleven_theme_options_add_page() call and fork that and theme_options_render_page() to extend it with additional options.

Also I think that unlike core Twenty Eleven wasn’t declared to be ready yet. Things might change.