Waterfalls and plunge pools for wild swimming – up[ by Ulpha is good.
Beatrix Potter’s house is nice for the little ones.
Dalton Zoo is good but take a packed lunch as the cafe ain’t all that!
Muncaster Castle and Ratty’s Railway are good if you are over the West side.
Dare I say the pencil museum in Keswick.
Rheged near Penrith is worth a visit, especially if its wet.
I quite liked Brantwood on the east side of Coniston where John Ruskin lived. Food is good, house and paintings are good if a little dry for the kids but the gardens and area around are great to explore with kids on a fine day.