Thought I'd provide a list of lesser known sites to visit in Sri Lanka (but which are equally amazing as the more well known ones). Many of them haven't made it to the regular tourist circuit and some of them are even unknown to many Sri Lankans as well.
-- Haththikuchchi (near Galgamuwa, on the way to Anuradhapura)
This is ancient monastery complex over 2000 years old (yes another one!), but this one is a really well kept secret with tonnes of ruins in sylvan surroundings. If you like also that Indiana Jones feeling and stunning views of the countryside from the top of rocks check it out. You also have to watch out for wild elephants while exploring.
-- Ritigala (off Habarana)
This is another ancient monastery complex but set in a total jungle setting and on a mountain (with fabulous views). It is criscrossed by ancient stone pathways and stairways and ruins that have been built on the mountain in thick jungle.
Picture of ancient stone pathway through jungle: flickr.com/photos/…
-- Vaedihiti Kanda (Kataragama)
Most people who visit Kataragama flock to the major Hindu/Buddhist temple that is found there along the banks of the river. However the vast majority pass by Vaedihiti Kanda which is a mountain nearby with a Hindu temple at the top. Again this is in a jungle setting and to reach it you have to climb through a jungle teeming with monkeys. Best to go when there is a pooja (religious offering) taking place, it is a beautiful ceremony. Don't carry conspicuous food as the monkeys snatch it from you.
-- Namal Uyana (Ironwood Park) Galkiriyagama
Another forest monastery on a mountain!! This time in a huge forest of iron wood trees. Everywhere you look it's ironwood trees. But besides the ancient ruined temples in the ironwood forest it is also home to one of Asia's largest pink quartz mountains. Apparently a lot this pink quartz was shipped off to adorn parts of the the Taj Mahal.
-- Mulawella (Sinharaja Rainforest)
No this is not a monastery but a jungle mountain which is provides excellent treking through virgin rainforest and provides absolutely breathtaking views of the surrounding countryside like: http://2.bp.blogspot.com/_uqdOHNsrdIM/S0W7HgTs44I/AAAAAAAADY0/Kk4ZFRzvKiQ/s1600-h/a+119s.jpg - unknown to many Sri Lankans. This is in a nature reserve and you need a permit to enter.
-- Pidurangala (Sigiriya)
This is Sigiriya's totally ignored cousin, which hardly anyone visits, but the view from the top of this rock provides amazing views of sigiriya itself (as well as the rest of the countryside) as well as plenty of caves
MORE INFO: http://www.brelief.org/reports/piduran.html