If you're wanting to visit Castlerigg then Keswick is your best bet.
Otherwise, further north, I would have been inclined to bring you off at Junction 41, and put you in Caldbeck for the night.
I would do Castlerigg at sunset on arrival night.
Then on departure morning, take the first Penrith bus at before 6am as far as Threlkeld.
Ask the driver to set you down at the railway path, just as he turns off the A66. Then it is just over the hour, at my pace, to walk back to Keswick old railway station on the old railway line. This morning it was just Sun rise at that time, a beautiful sun rise. That walk would have been magical today. It is at any time. Red squirrels are a common site on the path along with other wildlife.
I can't remember the name at the moment but just at the end of Station Road facing the war memorial there is a restaurant which does a fantastic breakfast (pre book - it is very popular) or Bryson's Bakery or any of several cafes in the Market Square.
Drive out of Keswick on the A591 along the back of Bassenthwaite Lake, then right at Bothel onto the A595 then the Carlisle western bypass to take you to the M74 for Glasgow and the north. On the way along the lake park at Dodd Wood and take the short lake path past Mirehouse Mansion to the idylically situated St Bega's Church on the lakeshore.
Take a comfort break for early lunch off the M74 at Moffat.
Coming in from Liverpool, coming off at junction 40 I would say take the A6 South to Eamont Bridge for the prehistoric stuff, then the A592 along Ullswater, calling in to Aira Force, then up the Kirkstone Pass. At the summit turn right down 'The Struggle' to Ambleside, up the A591 calling in to Grasmere for your Gingerbread, then back on the A591 up Dunmail to Keswick.
Check in to your hotel.
Then do the classic circular drive- Portinscale, back of Derwentwater past Catbells, over the Honister Pass to Buttermere (for the Ice Cream parlour), on to Lorton and back over the Whinlatter pass to Keswick.
After Castlerigg either go to the Theatre by the Lake for, well, theatre. Or the lovely old Alhambra Cinema for a movie. A proper family run movie theatre.