If I've got exactly one day to show someone the Lake District properly, I structure it around light — how it changes across the landscape from dawn to full dark — rather than around a list of places. Here's how that day actually works.
I'd start in Borrowdale, one of the most geologically layered valleys in the region, using the morning light low and raking across the fellside to actually point out what's shaping the landscape — glacial U-shaped valley profile, scree slopes still actively moving, the visible line where volcanic rock gives way to slate.
Honister Pass, connecting Borrowdale to Buttermere, deserves more precision than it usually gets. It's the steepest motorable pass in the Lake District — not just "one of the highest." The slate quarried here has been worked for centuries, and the exposed rock faces around the pass are a genuinely good place to see Lake District geology laid bare.
Buttermere and its surrounding valley are quiet enough, especially away from the main lakeside path, to actually see wildlife rather than just scenery — red squirrels in the surrounding woodland if you're patient, a good range of upland birds on the fellside above.
The Lake District has exactly two officially accredited Dark Sky Discovery Sites: Low Gillerthwaite Field Centre in Ennerdale, and Allan Bank in Grasmere. Other valleys — Wasdale, Buttermere itself, Langdale — are genuinely excellent for stargazing, with very low light pollution, but don't carry the formal designation. If your day started in Borrowdale, Ennerdale is a reasonable evening drive and closes the loop nicely.
Check the moon phase before committing to a stargazing evening — a bright moon washes out everything except the brightest stars and planets, regardless of how dark the site officially is. Aim for a few days either side of new moon.
This route can be built into a private, fully customised day. Enquire about bespoke touring.