Driving Highway 3 between mid July and mid October? A must-do is the drive up to the Cascade Lookout, accessed via the paved 7.8km long Blackwall Road. The views of the valley are spectacular.
Blackwall Road continues 6km after the lookout and leads to alpine meadows, which come to life with wildflowers in late July. If you have the time and your own vehicle (not a rental, as the road is not paved), I'd recommend the side trip.