From the Nisqually (southwest) entrance to Mt. Rainier National Park, follow Hwy 706 past Longmire towards Paradise, turning off for Ohnapecosh. Continue on a short distance to a roadside parking area at the Reflection Lakes. On the opposite side of the road is the Pinnacle Peak trailhead.
From the trailhead at Reflection Lakes (4900'), follow the Pinnacle Peak trail upwards for ~1.3mi, gaining 1100'. The maintained trail ends at the Pinnacle-Plummer Saddle (~6000'). From here, bear right/west and pick up a user trail that heads up the ridge and across the north shoulder of Plummer Peak. Another trail breaks left along here heading for Plummer's summit, but continue on straight ahead, skirting a small tarn in the early season and pick up an obvious user trail. The trail descends to meet Denman's rocky south spur ridge. It's possible to scramble up through here, but it's easier if you continue along the trail past a scree field on the right. Leave the trail at this point and scramble upwards (class 2-3) a short distance through trees and some loose rock to the ridge and continue up to Denman's broad summit (6006') marked with a large cairn.
Might like a helmet for Denman - although rockfall danger is probably pretty minimal.