Twin Falls trail is a moderate hike with some decent elevation & a few good climbs.
Warning : parking can be Extremely difficult. Even with a Discovery Pass, unless you get there early - expect to park pretty far down the road from the trailhead.
Warning #2: there are TONS of families/little kids on the lower half of the trail (up to the falls). Expect delays. Trail running cam be pretty tough here.
Once past the falls... It opens up nicely.
From the top (simply a gravel road), approx. 1/4 mile due North is an unmarked entry for an unmaintained trail up to Mt. Washington. Haven't done the entire thing yet (2hrs up from the unmarked entry), but the first part was awesome. Will definitely go back to hit the summit.