After completing the game, several bonus items will unlock--a couple of DVD-style extras, cheat codes, playable Heroes, and an insanely-difficult level.
Available Heroes: All
After beating the game, you're able to play any of the previous levels as any member of the Fellowship, save Boromir--you get his brother Faramir instead. We highly, highly recommend taking a level 10 character into the Palantir, unless you'd like a real challenge. If you have a friend handy, taking these levels in co-op mode will ease some of the pain.
Saruman's Palantir consists of twenty waves of increasingly difficult enemies, in a small, misty area--with no Checkpoints. And when we say 'increasingly difficult,' we mean 'this level is nearly impossible to beat.' If you find yourself having an inordinate amount of trouble in Saruman's Palantir, you can ease your way through it by getting yourself some extra uses of your special ability. Use the ability right off the bat, and then let it recharge by letting the last enemy on each wave of attackers live and chase you around (it's especially easy with archers, as they'll just stand where they are and keep launching arrows. Hide behind the cover at the center of the level, and go get yourself a drink.). Good luck--you will need it!
The Palantir of Sauron level will unlock after you have raised one Hero from each Path to level ten or higher. Similar in form and feel to the first Palantir level, you'll face twenty waves of Sauron's minions. As with Saruman's Palantir, hold off on dispatching the last member of each wave to maximize your special ability.