Have a rest in the morning, eat breakfast, secure a moped and then head to Khun Korn Waterfall in the afternoon, maybe around 1 pm. The reason we're heading there in the afternoon is because the White Temple is perfectly on the way back, so that you can visit the waterfall and then make it back in convenient time to visit the White Temple for sunset.
You should arrive at the White Temple in good time so if you feel like a snack to replenish energy levels you can walk down the road to the side of the White Temple and get a mango smoothie from "Hugfarm Coffee". They're only 40 baht and one of the best I've had - they are AMAZING. Get one!
Don't forget to visit the Golden Temple within the complex of the White Temple and make sure to stay past 5 pm because after that all the people will clear and you might even get the place to yourself!