Ride to the top of Signal Hill on "The Big Easy" a wide, all-weather track that climbs gently for six kilometres gaining 420 metres. Half way up you pass a kind of hub called "The Plateau" where lots of trails meet. At the top there is a map board and a look-out over Dunedin.
There are several trails to choose from to ride back down to The Plateau. The Mistress is a good one to start on. It's intermediate, goes through pine forest and has lots of roots to keep you awake. The Jumps Track is fun; Haggis Basher is more challenging. If you don't have advanced riding skills you are probably best to ride back down The Big Easy.
Below The Plateau try The Mistress, Nationals, Student, etc. They tend more towards the downhill style of trail with lots of drops, ruts, roots, rock gardens and so on.
The trails are well signed and graded. You won't get lost and you'll soon pop out back down at the school.