Help / Information - travelling to Sri Lanka (Colombo)

Dear all, I am planning a one week holiday (March Protected content 28,) in Sri Lanka. I am travelling alone. I would be very grateful if you could give me some suggestions. I have already in mind what I wanna see, but the thing is, do I have to go through a travel agency or can I book the day I wanna travel ? Thanks a lot for all kind of hints :) Looking forward to hear from you. Cheers, Doris