You can visit pinnawala elephant orphanage
rafting is also available
beware of the spice gardens. they tend to sell medicinal things at exorbitant prices which are of no value
If you are interested in some Buddhist site please do not miss Anuradhapura (oldest Capital), when in Mirissa do go for whale watching, who knows you might get lucky. Mirissa beach hosts a free DJ party once a week (not sure Wednesday or Thursday but ofcourse you have to buy your drinks) Hope they still do...haha. Kandy is beautiful too, more like Udaipur in India. Galle is also not to miss. Banderwala is the place you can take a train to/from for amazing tea garden views. I loved this country. Enjoy.