So many! Depends how much climbing you are willing to do. I like to climb up Ullock Pike then across to Longside and Carl Side and see the views south and across Bassenthwaite. Whenever I'm up there, I nearly always seem to see fighter jets below me, going across the lake.
Alternatively a lap of Derwentwater is about 10 miles. For shorter, flatter ones, it's great to go around Buttermere but you do have the option of some climbing if you feel like it.
I don't know as much in the South Lakes so it might be worth a Google if you're at that end.
Enjoy beautiful Cumbria!