In reply to Thomas Hardbattle:
There's the classic Loop starting near Langleeford and taking in several big hills, Scald Hill, The Cheviot, Comb Fell, Hedgehog Hill etc.
Or there's some great lowland walks, places like Allendale where you can do an excellent loop taking in sections of the Issac's Tea Trail, the river East Allen and some old mining areas.
Some great coastal walks as well, Holy Island and not forgetting there's many castles up that coast line to explore and Hadrian's Wall.