Fairly spectacular waterfall with a 65 foot drop set amongst what was once fairly cultivated parkland. A pleasant (though relatively steep) walk to the top of the falls from a car park (free to NT members). There's a network of trails nearby and some lovely views.
Get there via Kirkstone Pass on the A592 from either Windermere or Ambleside - Aira Force is a bit past Glenridding. Or from the A66 between Penrith and Keswick, take the A5091 through Dockray.