[% FOREACH thing = env.keys.sort %]
    
    <li><b>[% thing %]:</b>[% env.$thing %]</li>
    
[% END %]
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so you were close .. few points:

• case matters (unless you explcitly say otherwise w/the ANYCASE option -- see docs)
• check the docs http://www.template-toolkit.org/ for the loop syntax and other control statements (IF, etc)
• note the $ that's needed in [% env.$thing %] -- without it, you're getting the key 'thing' and not the key that matches the value of thing.