You should swing by Jiu Zhai Gou as I was told by locals that July is the best time to be there although i was there in October last year. Its beautiful serene lakes (the movie Hero) and also a little bit of "Mountain" people along the way, do stay at the JuiZhaiGou Paradise, its a really beautiful hotel. Journey from Chengdu is 10 hours bus ride (scenic and farms) but one/two day at Jui Zhai Gou and another around the area (multi-layer lake), blueish green lake (some other colours) and waterfalls.
Intend to go back to JuiZhaiGou, Mongolia and Tibet in a single trip but looks like would have to a long, long break for this to work. Just Siem Reap (14-18 APR) and Palau (11-15 MAY) for now.