Despite being just 80 acres, Obstruction Pass State Park boasts a variety of trails. It's a relatively straight shot to the camping area and beach if you take the 0.6-mile central trail (it's 1.2 miles roundtrip). Or, you can take the 0.4-mile, very faint Sound View trail, which loops along a bluff before reconnecting with the main trail to finally deposit you at the camp and shoreline from the west.
But the best way to experience the whole trail is looping the central trail with the Highlands trail, making for a 1.5-mile loop down to the water and back up through upland forest along the northeast border of the park. Plus, this way, you get to take in all the informative interpretive signage provided by the park.
Set off from the parking area by walking through a stile. The junction greets you on the other side of the stile. Of course, it's possible to do the loop either way, but counter-clockwise gets you to the water faster, and makes for a slightly less-steep return. Choose the path straight ahead of you to do the loop this way.
The way begins descending almost immediately, and you'll be walking through an open forest populated by Douglas-first and western redcedar, and madronas. Pause and take in the interpretive panels as you hike; they give you context and information about the area, and might help you identify the plants and geologic features unique to the islands.
Continue descending. You'll notice the salal (a common ground cover here) getting taller, and suddenly will pop out into a campsite! There are 10 walk-in campsites available here on a first-come, first-serve basis. A surprisingly picturesque privy rounds out the scene—it's an idyllic campsite; perfect for introducing kids to backpacking.
The trail continues through the campground and to a fence on a bluff, affording you views of Obstruction Pass itself, as well as Blakely Island across the way. Turn left, and hike along the bluff. One trail offers access to the beach, which you can head down to explore before continuing on your loop.
Scramble up a small, somewhat exposed section of rock (the only somewhat hard-to-follow section of trail here) and look for a large brown sign with an arrow pointing into the woods. This is where you turn left and follow the trail inland.
Now on the Highlands Trail, begin a gentle climb. Look for deer in this section, as they like to browse in this sort of landscape and the Highlands Trail is less trafficked than the main route. If you see them, keep your distance. It's best to leave wildlife be in their habitat.
The trail mostly climbs, though there are a few dips to keep it interesting as you round the northeast corner and the trail turns left to return to the parking area. Enjoy the flat traverse here, and the final descent to the first junction, and soon after, the trailhead.