There is plenty to see at Yala. In a couple of days we saw golden jackal, leopard, monitor lizards, crocs, ruddy mongoose, chameleons etc. My tip would be to search in and around the track itself since the vegetation i spretty hick. Your best chances of sightings are in and around the track.
In all the enthusiasam to see the big cats and other animals do not miss out on the opportunity to spot some migrating and endemic birds in the Yala Sanctuary. It is teaming with birdlife and you can easily spot 50+ species if you spend a day or two there.
There is a particular spot that has crocodiles, so when at Yala ensure yous top at htis place and get your shots of the crocs. their camouflage is excellent, so spotting them itself is a good experience in itself! :)