The canal was cut in the seventeenth century to link the Atlantic and the Mediterranean without going round Spain, and it is a genuine feat of engineering rather than a scenic afterthought. The canal's history and engineering is worth a read, particularly the locks and the way water was brought to the summit.
Walking is the simplest option. The towpath is flat, shaded by plane trees for much of its length, and you can join it in Carcassonne and go as far as you like in either direction before turning round. An hour out and an hour back gets you well clear of the town.
Cycling covers far more. The towpath surface varies from good tarmac to rough gravel, so a hybrid or a mountain bike is more comfortable than a road bike. Hire is available in town and several companies will take a bike back from a downstream town if you would rather go one way.
Boats can be hired by the day or the week and require no licence. A day boat from Carcassonne will get you through two or three locks and back, which is plenty for a first attempt: working a lock is more entertaining than it sounds and slower than you expect.
The plane trees are the thing everyone remembers and they are under threat from a fungal disease that has forced the felling of long stretches. Replanting is underway with resistant varieties, but some sections that were tunnels of shade twenty years ago are now open sky.
