A glade is an open space in the forest, similar to a clearing.
A grove of trees is a group of trees together, usually without any undergrowth.
His forHeath (landscape)
Hills are small mountains, and have a much gentler slope than mountains do.
The horizon is the place far in the distance where the ground meets the sky.
Icebergs are huge chunks of ice that break off from the ice collected around land, or from glaciers that reach the sea. They float, but most of them is underwater and you can't see it, which has coined the phrase "tip of the iceberg" meaning that you can only see a small part of something and the problem is actually a lot bigger than it seems.
An island is a piece of dry land completely surrounded by sea. Some islands are very small, and are little more than a rock sticking out of the ocean. Some islands, like Australia, are very, very big.
A jungle is a rainforest that is particularly thick and has a lot of undergrowth. It is usually impossible to walk through a jungle, you need to bring a machete to cut through the vines.