I thoroughly enjoyed Chitwan, and I guess like the vast marority of visitors I too did not see a Tiger.
The highlight for me was a canoe safari, during which we found ouselves with a couple of metres of a Gahrial, a very rare fish eating crocodile with a peculiar long and slender snout/mouth full of evil looking teeth!
But what really made this trip for me was the half dozen Nepalese ten year olds in the canoe with us - their enthusiasm, sharp eyes and knowledge adding incredibly to the experience.
When I talked to our guide about this later, he told me it was part of their education, aimed at making sure the next generation both apreciated and understood the park, because one day they would be responsible for its care. How many other WHSs have this much forsight?