As Zenboy says Hardcastle Craggs, owned by the National Trust, is very nice it's also a relatively flat walk. Alternatively you could go up towards Stoodley Pike, which sits high on Erringden Moor between Hebden and Todmorden. A walk through Colden Clough towards Jack's Bridge and return would take you past Lumb Bank (a former house of Ted Hughes) and the village of Heptonstall (high above and older than Hebden Bridge) where Silvia Plath's is buried.
If you look at
http://www.bing.com/maps and select OS maps from the first selection box, you'll see that the whole area is criss-crossed with public footpaths.
