Secret tower level
In the third Human mission, where you need to escape a prison with the Elves, there is an entrance to a secret level. Around the horizontal center of the map, there is a winding corridor with several rooms you need to go into in order to get the power-ups and equipment. Look closely around the middle of the map to find a room. You need to bust through a door to get inside. The room has three different colored sheep surrounded by energy. In front of them are push-panels. When they are pushed, the corresponding sheep will say either "Bah", "Ram" or "Ewe". Push the panels in the following order: Bah, Ram, Ewe. You will get a door to a secret tower defense level after that mission.
Secret Orc level
Select the "Custom Campaign" option at the menu, then choose the "Exodus Of The Horde". The game wukk launch a screen with the selection of the first level. This extra campaign takes place after Thrall leaves Lorderon, but before Kalimdor.
Grank The Rat
In the fourth Night Elf mission, Wrath Of The Betrayer, at the top left side of the map is a small garden and a fountain that only Maiev can visit. Use your Blink-spell to get to it. In there, you can find a rat with the name "Grank the Rat", which has 100% Evasion and Spell Immunity. Therefore, you cannot kill it. Also in the Night Elf mission where you have to escort the runner to the escape point, in the upper left corner of the map near the escape point is a creature named "Grank the Rat", who cannot be killed by normal means. In order to kill Grank, you must continuously click on him until he explodes. He will drop a Talisman of Evasion which supplies a 15% evasion rate.
The Pandaren Brewmaster
Play the Night Elf mission where you have to escort the runner. In the lower left corner in the very back of the map is a Pandaeran relaxation area. A Brewmaster is here. He will spout some gibberish and drop Slippers Of Agility.
BloodFeast The Abomination and The Zombie Master
On the Undead mission, The Flight From Lordearon, you can re-summon a Zombie Master and BloodFeast the Abomination near the end. On this mission you find a Staff of Animate somewhere (not hidden). You will find a room with Necromancers summoning something, and a few enemies such as Ghouls and Crypt Fiends. Kill them, then go on the pile of where the Necromancers were summoning and use the Staff to summon a Zombie Master for sixty seconds to help you fight. BloodFeast the Abomination can be summoned the same way.
In the fourth Undead mission, The Return To Nothrend, you will find a giant penguin in the left corner on the island that you started on. Kill the guarding sea lions and destroy the tent so you can approach it. Then, kill the penguin itself. You will get a Ring Of Superiority. A normal penguin will spawn next to it. You can kill it, but it will not give you anything.
Destroy huts, tents, or other homes. Some of them will have hidden items such as Scroll of Town Portals, Potions of Mana, etc. Pick them up for later use.
Drunken Panda picture
Successfully complete all the levels in the "Secret tower level" to see a picture of the Drunken Panda along with what seems to be his daughter. Then in the next chapter, you can use him to help.
In the prison mission, you will find various humorously named guards, such as one called Trogdor. It is interesting that he is burninated almost immediately after you see him. Other guards will have names like Seth, Marcus, or Troy.
In the last Human mission, Lord of the Outland, where you need to assault the Lord's castle, you can find the Butcher and Wirt's other leg (from Diablo) just northwest of where you start. The Butcher will also say the same sentence, that he did in Diablo, "Fresh meat".
WarCraft 3 reference
In the first Undead mission, King Arthas, you can find Little Timmy from the original WarCraft 3. He is in the woods, left of the town which is northwest of the three Altars. Little Timmy will sell you some Ice Shards, which are useful both later in the game and the campaign.