Keswick is NEAR (though not ON) Derwent Water.
It is short walk from the centre of Keswick to the lake edge (down Lake Road), and the lake side is a lovely place.
I seem to remember there is a cafe at the lake side, and you can feed the ducks. Nice views so plenty of chances for photos.
Scafell Pike towers over the town of Keswick so plenty of opportunities for some great photos of rooftops with the hills in the background.
There MAY be a bus in Keswick that takes you on a tour of the local area. Mountain Goat do run bus tours.
http://www.mountain-goat.com/
There is a Tourist Information Centre in Keswick so they will be able to help find you something to do
http://www.keswick.org/