I visited in January. It was amazing, despite being during the presumably off-peak wildlife viewing time, camping, or swimming season. Don't think of the island as only a summer recreation destination. It has wildlife viewing, hiking, and history available all year.
I saw all of the wildlife I had hope to see except bighorn sheep while driving to and from the Ranch! Northern shoveler, bald eagle, porcupine, pronghorn, bison, mule deer, and many small birds. It was an amazing few hours.
There are many trails on the island. There are trails for all types of hikers and all distances. Ladyfinger Point is 1/4 mile but takes you out to a fun outcropping of rocks to climb on and a great view of Egg Island, a huge rookery. On the other end of difficulty is the 6.5 mile, 2000 ft change in elevation Summit Point trail. There are trails for quick family drive-thrus and trails for avid hikers that are interested in spending all day on the island.
The Ranch has a fun tour through history with a pamphlet you can pick up. At the Ranch you can also book horseback tours of the island. I do not know if this is offered year round, it didn't seem to be open in January.
Park Rangers offer tours of the hydroelectric powerhouses daily. There are two visitor centers, one accessible from the Oregon side, one accessible from the Washington side. Both visitor centers have fish viewing windows but there are not always fish to view! The salmon runs are seasonal, with the best viewing in the spring and fall.
Delicious food in large helpings. The decor is a little dusty and the paint is chipped on the wall. Very convenient because it is inside the Quality Inn. Many locals were eating here, which is always a good sign.