The most common use is the east coast. Between May and September, when the south-west is wet, an eleven-day itinerary can run the Cultural Triangle and hill country as usual and then cross to Trincomalee or Pasikuda instead of finishing in the south. That crossing is a long drive out of a ten-day trip and an easy one out of eleven, and it puts you on a coastline most visitors never see.
The alternative use is simply to stop. Four nights on the south coast at the end — rather than the usual two — changes what the trip feels like on the flight home, particularly if you have been moving hotels every morning for a week. Several of the itineraries below are built this way, with a touring first half and a stationary second half.
Eleven days is also enough to see both ancient capitals, walk Horton Plains, and still fit two separate safaris — Yala or Udawalawe in the south and Wilpattu or Minneriya in the north — which is the point at which a wildlife-led trip starts making sense as its own itinerary rather than a set of add-ons.