Climbing crags - nothing I have seen, but I never looked at that part of the country with that in mind. It is really really FLAT. Not without its charms, but hiking or biking is the best you can hope for.
Helgoland - a small island off the cosat - has some sandstone towers, but my guess is they are off limits for climbing.
Again, you'll have to venture south - but even with public transport, it's really not that far. Also north is reasonable as well (Norway, Scotland, etc.