Sri Lanka offers a range of diverse destinations and attractions to visitors. You may already have a general idea as to why you want to visit Sri Lanka.
You can gather further information by surfing the net and reading descriptions provided my travel agents in Sri Lanka, travel forums like trip advisor is a great place to start, you can also look at investing in a travel guide book such as lonely planet.

Narrow your search to what exactly you want within the time frame you have. Again you can get in touch with a few travel agents in Sri Lanka (through their websites) and ask them for advice and guidance.

Its best to contact travel agents from Sri Lanka as they have a better idea of the market conditions locally and what you can do depending on the time of the year.  (for example Lonely Planet guide recommends glass bottom boat rides off the coast of Hikkaduwa. We do not recommend these rides any more as the diesel from the motor boat have destroyed many of the coral and the ride is virtual useless now)
Sri Lankans in general are very helpful and will certainly go out of their way to be of assistance and help… and in some instance provide more information that you may wish for! The travel industry is no exception. They are eager to provide information and guidance, even without a sale.
Just from experience the range of travel activities can include anything from a relaxing beach holidays, eco and wild life tour, cultural & heritage tours, spa retreats, Medical tourism packages, Golfing breaks, a variety of soft adventure excursions, mountain trekking, whale watching, weddings and honeymoon packages, or a combination.

You can then decide as to how best to book your trip. You can book it through an agent in your country or through an agent in Sri Lanka (where you can either ask your local agent to book you airfare or I would recommend checking for airfare rates from an agent in Sri Lanka too. It could work out to be cheaper).

Either way you book your travels, I strongly recommend you book and organize all aspects of your trip prior to arriving in Sri Lanka, including your airport transfers. You do have airport transfer services at the airport, but somehow I feel you’ll be more comfortable knowing someone is waiting to pick you up on arrival.

If you require any guidance from my end please do not hesitate to holla!