Spiti in 6 days is possible but it will feel rushed, especially because of long travel distances and high altitude. If you still want to do it, the best way is to take the Shimla route for entry and exit via Manali, as it helps with acclimatization and saves time on return. A tight plan would be Shimla → Kalpa → Kaza (2 nights) → back via Manali, covering places like Key Monastery, Kibber, and nearby villages quickly. Just keep in mind it will involve long drives and less time at each place, so if possible adding 1–2 extra days makes the trip much more comfortable.