Don’t even think about trying to do Tibet in a long weekend. Since it takes at least 3 days to recover from altitude sickness, it’s a destination that requires at least a week to experience properly.
One Week
Take in the food and shopping of the Barkhor for the first few days, then tour nearby Sera and Drepung Monasteries. If you feel up to it, spend the next day at the Ganden Monastery. End the week with a morning visit to the Jokhang temple.
Two Weeks
Add to the above: an all-day, scenic journey up the stunning Khamba La Pass followed by 2 days in Gyantse and another 2 days in Shigatse. Return to Lhasa for the remainder of your visit.