How do I return ALL entries that are singles
There's no such method available for craft.entries, at least it's not documented. But you can use craft.sections to get an array of section models and filter them with a conditional within a loop:

{# Get all Section #}
{% set sections = craft.sections.getAllSections() %}

{# Loop through all single sections #}
{% for singleSection in sections if singleSection.type == 'single' %}

    {# Get the first (and only) entry of that section #}
    {% set singleEntry = craft.entries.sectionId(singleSection.id).first %}

    {# Output #}
    {{ singleSection }}
    {{ singleSection.isHomepage() ? '(Home)' : '(not Home)'}}
    {{ singleEntry.dateUpdated.atom }}

{% endfor %}