Island Center Forest made for a very easy and very pleasant hike to fill a couple hours. There was minimal altitude gain, so it would make a great hike for children or those with physical limitations. The trails were overall in very good condition, with only a few boggy spots to navigate through. There were not even any fallen trees to contend with. There were a couple piles of horse manure along the way, as the trail is also open to horseback riders. In fact, I encountered two of them while I was out.
The trails are generally well-marked at intersection points, although there are several spots where I'd come to a fork in the road and there would be no sign to indicate which way I should go. As such, I'd highly recommend taking one of the free trail maps that are located at the trailhead (I started at the 188th Street trailhead).
Overall, it's a pleasant walk through the woods and makes for a good reason to explore Vashon Island if you have a free afternoon.