Rats can eat most things. Read this Rat diet, especially the forbidden foods section.
Citrus peel or seeds - Peel never. Seeds definitely not for male rats, can cause cancer. As a side note, never feed apple pips, they contain small amounts of cyanide.
Meat bones - a big fat yes, they'll love you for it
Stale bread - yes, but not mouldy
Tea - mine steal tea all the time, though make sure it isn't too sugary or milky.
Meat fat - waaay too fatty for them, though they will gladly eat it.
Plant leaves - I wouldn't recommend it, they'd be more likely to build a nest from them.
Bark & Twig - yes, but only if properly cleaned and driedas these can bring in microscopic parasites.
Berries - nope, not unless they're human-safe ie blackberries etc.