Nice campsite within walking distance of the castle ....and pay and drive forest Drive ....

https://www.forestryengland.uk/kielder-castle/kielder-forest-drive

(£3 to drive 12 miles of fab forest drive)

Kielder observatory

and otterburn ranges ....they are open to public access at certain times when the military arent busy up there and I'm sure some green lanes cross them .