You could try Ullswater. There used to be a good hotel with excellent food on the North Shore, Sharrow Bay I think. Then there are various gentle walks on that side or more serious stuff further west or on the south shore.
You could stay in Grasmere and walk from the hotel a different route every day.
We usually stay at Rock Lea in Windermere (great breakfast) and then either walk into Bowness, lunch at the Tapas bar Bodega go for a lake cuise then have dinner at Sutherlands or the Old Schoolhouse, or walk to Ambleside and lunch at Wateredge hotel or the Glass House, and then catch the open top bus back.
You may want to be sure you pick the right time of year to go unless you like huge crowds. You've been there before, so you may already know this. When my wife and I went there in late June, the traffic was practically bumper-to-bumper. Going there a month earlier would have been cooler but also much less congested. Just a thought.
Definitely agree with plankton. Also, avoid the school hols in July/August.
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