In Siem Reap you can rent a tuk tuk from 4.30 till at least 14.00 for $20. When you’ve a tree day ticket it’s a nice option to rent the Tuk Tuk the second day. Wake up early see the sunset have breakfast and visit the temples when it’s not that hot yet. We did the big temple round tour (big loop) skiped two temples to see the Tomb Raider temple.(normaly in the small loop).
Drink enough water, and take it easy. Tip: Pay the Tuk tuk after the ride. In Cambodia the people are very kind but you never know.