Three weeks or longer gives you a chance to explore the sites in greater depth and to get off the beaten track. You could easily spend a week exploring the villages of the south coast, or a few days trekking in the hill country, visiting some of lesser visited ancient sites of Northwest and North Central Provinces, or exploring the island’s wildlife and birdlife in greater depth. Accessible once more, the unspoilt beaches and wild country of the east are an increasingly draw for travellers, with laid back Arugam Bay the island’s latest ‘cool’ beach destination or Sri Lanka’s version of Surfers Paradise. You could also see a very different side of Sri Lanka by visiting the Tamil heartland of Jaffna for a few days, which is simple breath-taking and starkly different to the rest of the island.
I strongly recommend a eco trek or visiting a local community. If you are up for it we can organising a stay with a local village family, where you will cook with them, go about their daily lives and even help in the daily chores. This will be organised free-of-charge while you pay for what you feel fit to your host. You will learn so much about their simple existence but in return help them with your knowledge and help them with economically.
A 3 week Tour of Sri Lanka will give you amble time to explore the length and breadth of the island. Ask you travel guide for a suitable itinerary based on your interests.