The River Greta has approximately a 5 mile canoe-able stretch that you can use 365 days a year as long as the water level reaches the white mark on the bridge to protect the salmon beds.
For route information check out the link here. There is an access agreement that you have to adhere to. To see this click here. The disused railway line runs along the river and is a great path for walking and cycling with pretty views.
Fishing on the Greta is another favourite past time here, permits are required and all the relevant information has been covered by people more knowledgeable on this subject than I am. So click here to find out what you really need to know about fishing on the River Greta.