With four to five days, I'd suggest that you first decide on what are your interests: are they relaxing on the beaches or shopping or cultural sightseeing or active holidays with rafting etc?
Based on that the following can be options:
(1) Shopping: Colombo and the outskirts of Kadawatha, Maharagama etc as well as Kandy
(2) Beaches: Southern end of Sri Lanka - Tangalle, Matara, Weligama Bay
(3) Cultural sightseeing - Anuradhapura (the Sri Maha Bodhi tree - the ancient tree that is the most important in Sri Lanka as it is believed to be from the sapling gifted by Mahinda and Sangamitha, Emperor Asoka's children to the King Thevanampiyatissa and the numerous ruins and the stories behind them are very interesting, provided you find a good guide), Polonnaruwa (the still used ancient tank which is still the largest tank in Sri Lanka and several ruins), Sigiriya, Dambulla and Kandy
(4) Sportive holiday: White water rafting at Kukuleganga and Avissawela or Yala safari or mountain climbing
All the above can be managed in any range of budgets if you choose the accommodation within your budget.
For some more information, do visit the Sri Lanka travel guide at Travellerspoint... there is quite a bit of information that can be useful for your group.