Thoroughly recommend starting as early as possible in the morning (park opens at 6 am) to avoid the heat and bus loads of package tours. A cycle ride the inner wall section is very pleasant in the early morning, flat and cool. For out lying temples a car or Tuk Tuk is more comfortable. 3 hours is enough for me before I get temple fatigue - so recommend stick to a few temples. My shortlisted favorites were:
-Tra Phang Ngoen
-Sorasak (a small chedi flanked by restored elephants)
-Si Chum , stunning standing Buddha (opens 7.30 to 17.30)
-Saphan Hin - in the jungle, steep flight of stair up to a Buddha with great views (sunrise spot)
Best time of year is definitely from late Nov, after the rains, when it is green, the moats and ponds are full, water lilies in flower and it is cooler. The Loy Kratong festival is a fantastic time to come -for 4 days before the November full moon there are festivities during the day and all the temples and ponds are lit with candles by night.