You pass through the park on I-93, so waymarking the entrance at either end is a little iffy (stopping on the shoulder of the interstate highway to photograph a "Welcome" sign). There is no traditional entrance to a park; in lieu of that, coordinates given are for the Visitors Center at Flume Gorge.
On the highway, you're in the park for about 8 miles, end to end, with opportunities to stop at the Cannon Mountain Tramway or, in season, skiing at Cannon Mountain (both state-owned). Enjoy swimming and camping at Echo Lake, and natural wonders throughout, such as Flume Gorge, The Basin, Boise Rock, and a good look at where the Old Man of the Mountain used to be.