The monument is not a single structure but a collection of mounds built by the South mound built by the South Appalachian Mississippian people about 1,000 years ago.
This particular mound is the Great Temple Mound and you are able to climb to the top.
It is a sacred site and there are a lot of things you can't do on the grounds including flying kites.
The view from the top show the floodplains of the Ocmulgee River and on the opposite side you get a great view of the Macon skyline.
I'm always a sucker for a gift shop and National Parks have some of the best gift shops anywhere. Ocmulgee's gift shop didn't disappoint as they had complimentary packs of of trading cards featuring historical figures and events related to the park. Here are some scans of the front and back of each card in the set with my favorite being of General Sherman.