A clickable (or "Clickable Treasure") is an item that can be clicked on to get a bonus, like gold or extra regenerated Energy. They have a chance of appearing on either side of the path where the player's hero is fighting. Currently, there are 5 types of clickables that can appear.
The name says it all; the Orange Fish is a clickable that looks like an orange fish. When clicked on, it will award the player with gold and rubies. The number of rubies is random and the amount of gold scales with the player's current zone and can be increased by buying equipment with +x% Clickable Gold bonus.
The Gold Pile looks like a pile of coins and, in contrary to the Orange Fish, awards the player with just gold. This amount, similarly to the Orange Fish, scales with the player's current zone and it can be increased by buying equipment with the +x% Gold Piles bonus. It is also the most common clickable found.
An Energy Pillar is a yellow pillar that regenerates 25% of your total Energy when clicked on.
A Mana Pillar is a blue pillar that regenerates 25% of your total Mana when clicked on.
The Magic Clock is a Grandfather Clock that doubles game speed (not affecting haste) for a short amount of time when clicked.
1Excluding "Gold Pile", the names used above are not official.