right - so you'll probably stay at the coast, but when you get there you should book a couple of days in the interior - we went to the elephant orphanage on the first day of this. the actual elephant riding was a place just near the orphanage that our driver asked if we wanted to go to. he probably took a kick back for taking us, but it was an wonderful experience and anyway, that's how sri lanka works - kickbacks and tips. anyway, then the elephant orphanage proper. we went at the right time to see them all troupe down to the river and bathe. just brilliant. Then we saw a tea plantation and then we went to Kandi, which is the historic/cultural capital. we did some sightseeing (botanical garden, temples) and saw a cultural show (dance/masks. brilliant stuff). next day we saw some temples carved into the mountains, and a really old palace, also carved into a hillside. very impressive. our driver also took us for a massage - really strong, almost brutal - and to see a spice garden (which was a big old sales pitch).
if you can, avoid being taken to spice gardens and jewellery shops. you'll get fairly high pressure sales. the drivers all get paid for taking you there, so the way to avoid it is, if they mention it, say - "i'll pay you X amount if you don't take us to any of these places". we did that towards the end of our trip. much easier. You will end up tipping loads. it's how the economy works and these people are very, very poor. try not to resent it.
we also went on a couple of trips by tuktuk - which is a great way to experience the feeling of the place, if a little hair-rasing (sri lankans drive like loons, it's all about the overtaking). We met a tuk tuk driver on our second day who took us to the nearest town. He suggested an excursion to a nearby waterfall the next day, which was great. Then we negotiated a day trip with him. We went to Galle, which is an old colonial town. Really lovely atmosphere. On the way we went to a placve with a turtle conservation project - they hatch hundreds of baby turtles a week. it was so cute! he also took us to the fish market - all the catches being sold early in the morning between fishermen and traders - not for the weak of stomach - and to a guy who took us on a small boat around an inlet and into a mangrove swamp. we saw loads of wildlife including baby crocodiles and monitor lizards. On the way back, and by my special request we also saw a museum of masks, and we bought one as our souvenir of the holiday.
most unlike me - but we got loads done. sri lanka isn't much for nightlife, and the tourism industry isn't slick and well-developed... but it's brilliant. so friendly, cheap (despite alllll the tips. honestly not overstating this element), and fascinating.
get a decent guide book and plan your own local excursions - make a deal with a driver. the centre (kandi etc) is probably best booked through an organised company, but it'll probably be just you and a driver in a a fairly beat up car (with air con!) anyway.