Ennerdale is the niceset valley IMHO but is very remote and quiet therefore maybe not what you’re after.
I would also recommend the NT campsite in Langdale as the Langdales mountains are quite iconic and it’s near the old dungeon gyll which is also quite famous.
You’d also be near to a number of tourist attractions and the towns of Ambleside or Bowness. Ambleside would be my choice as it’s got good outdoor shops and some easy walks (eg. to the waterfalls or Loughrigg) and I just generally really like Ambleside, but Bowness is perhaps better if you want other types of shops.
A boat ride on lake Windermere is something that everyone seems to have to do.
Keswick is also good for a weekend away and there are a few good campsites near, my fav was always the one in Braithwaite, although I’ve not been in a while.
I really love the north lakes, but find the south lakes better for none outdoors-y stuff, although maybe that’s because I know it better?
Is she into poetry, or fluffy bunny rabbits? 😉