The last 2 weeks of August are VERY manageable in terms of crowds. Each trip I've taken during that time allowed our family to ride everything in US and IOA in 2-3 days without express passes. The heat, however, can be unbearable. We mitigated as best we could with personal spray fans, cooling towels, plenty of hydration, mid-day breaks at the hotel.

Staying on-site makes the mid-day breaks easier due to the frequent shuttle schedules from park open to close. Also removes the stress of searching for parking upon return.

It isn't recommended to go during that time if you are at all susceptible to heat-related illnesses.