SWAT Chase: Envelope D
You will enter the elevator shaft. Ride the elevator all the way down until it stops. Get off the elevator and wait for it to rise again. After it rises, jump down to the bottom and use first person mode to find the control box. Use a Batarang to disable it and the door behind it will open. Stay clear of the elevator and wait for the correct time to get in the open room. Inside the room is "Secret Key 2." Get out of the room and ride the elevator up to the top. At the top there is a door that is supposed to be locked. The key opens the door, and inside is an envelope with the letter "D" on it. Finish the game and the "Infinite Special Move Bar" cheat will be unlocked.
Press [Action] [Guard] to use flash bombs. They cannot be selected in the gadget/first person view.
Bridge: Envelope B
Knock out the thug at the start of the level. You should see the glowing item on the left when you start. Climb the ladder and jump/glide around the crate onto the other crate. Then, glide onto the platform with the letter.
Gasworks: Envelope E
You will reach a room with two levels, each floor with two goons on them. First, take out the goons. Then, look around on the lower floor until you see a sewer pipe with boxes in the sewage. Carefully glide down and land on the first box. Jump over the next few boxes and pick up the envelope with an "E" on it. Finish the game and the "Infinite Health" cheat will be unlocked.
Plant Electrocution 2: Envelope C
You will start the Plant Electrocution 2 level by sliding down towards some holes in the ground. The second hole is not fatal. Fall into it and glide to avoid minor damage. You will see a lit area. Glide down there and pick up the item you find inside. Use the grapnel to exit, jump out of the hole, and finish the slide. The item you picked up should be the key do the door right beside the vines behind the glass. Go inside and fight one of the "Plant Men". You will find an envelope that has a "C" on it in the room. When you finish the game, you can activate this cheat for all weapons and unlimited ammunition.
Rooftop Battle: Envelope A
When the clowns chase Mary, you will go up to the rooftops. Grapple hook up to her roof and beat up the white clown. Go up to the next roof, where you are on a little balcony. Jump across, then go in to see a thug run across a hall with the girl. Follow and he will jump off the roof. Follow, look down, and drop to lowest edge. Walk along to left until at the corner the grapple icon appears Grapple over, go in the door and pick up Envelope A.
Hacking into Issac Evers files
Find the second wire of the color you need that is not powered. You need to make the powered wire reach the non-powered one. For example, if you work on blue, you need to connect the live blue wire to the dead one.
Look in your inventory and examine the disc Batman picked up before fighting Mr. Freeze. It will reveal possible colors that work with the Blimp puzzle. The combination is Purple and Green. Make the color purple, "mix tubes", then make the color green.
FunnyBones Warehouse lock
Instead of trying the 999 possibilities on the lock, look for figurines like the one in the room near your starting location. There is another one behind trucks and one outside the fence when you get on the roof of the building with the first figurine. The combination is 619 from left to right.