If you want guaranteed wildlife sightings at Periyar you have to do one or preferably both of the following:

  1. Go on the early morning first boat ride of the day and book the Upper Deck seats.  The boat leaves at 6am or 7am depending on season, and you will most definitely see elephants at the edge of the lake, wild boar, deer and an assortment of birds.  Anything else is possible and your luck!  The Upper Deck is very important for a better view. The boat can get crowded in season so book in advance if possible.
  2. Go on a jungle walk accompanied [compulsorily] by a tribal guide from the information centre near the boat jetty.  The rates are extremely reasonable and if you pay the full charges you can go for as long or as short a walk as you want.  Chances of elephant sighting are high.  Guaranteed sightings of giant malabar squirrels, assorted deer, lion tailed macaque, black langurs, huge butterflies like the troides minos - southern birdwing which is the largest in India, helens, mormons etc and a wide variety of birds. Anything else and that is your luck.
Periyar is always pleasant even at the height of summer due to the 1600m altitude.  Just before the monsoons is the best time to spot wildlife.  Enjoy your trip.