The Lion Rock and Ancient City of Sigiriya
The Unesco World Heritage site of Sigiriya is a massive urban center dating from 477-495 AD. It was a sophisticated ancient city with water works, gardens, commerce, defensive walls and a palace on top the Lion Fortress. The trees are overrun with monkeys, but perhaps its most notable claim to fame is being featured in the 1982 Duran Duran music video Save A Prayer. If you haven't seen that lately, I'd look it up on Youtube--it'll bring back memories.
The hike to the top of the fortress takes about 45. It is fairly easy though there are lots of stairs. There isn't much on top except for water basins and foundations, but the view is incredible. Allow 2-3 hours to wander the entire site.