As with National Parks during peak season... Arrive early (before dawn); stay late; hike / nap mid-day.
Early Morning; Bear and Sprague Lakes (more wildlife, less crowds)
Afternoon / Evening:
Fall River and Trail Ridge road (and on over to Grand Lake) I prefer the west side of the park (more lush, less people)
(can come home in the dark! I used to do this run in summer evenings on my motorcycle, usually via WY Snowy Range (which are really nice alpine mtns too )
Ask at ranger station the day before for ideas and wildlife sightings. Lots of nice streams for a picnic lunch and afternoon nap.
For your route to Estes Park, make it a round trip, with one leg through Glen Haven, Big Thompson / Devil's Backbone (Loveland route) I prefer that leg headed westbound, but depends on your schedule, do it during the light of day! Coming up through the switchbacks above Glenhaven and into Estes from the Northeast is very picturesque (and near my childhood ranch home). GOOD cinnamon rolls @ Glenhaven general store, Just had one last week.
Highway 7 "Peak to Peak" is a great route (to the south) and really nice for Sept visits (fall aspen colors)
https://www.tripadvisor.com/Attracti..._Colorado.html
For other readers... when in Rocky Mtns, follow 'Scenic Byways!' Be prepared for 3-5PM rain and lightning (daily). Crisp morning air in Estes Park is great!