You could try Loughrigg Fell.
If you buy a 1:25,000 map of the area, you should be able to work out a suitable route from Grasmere.
Alternatively, stop in Ambleside on the way through and and pick up some of the leaflets describing easier walks, there are plenty about, Tourist Information may be best. They may be easier walks, but they should still give a challenging 3-4 mile run.