Bring water shoes to enjoy playing in the Ohanapecosh River. The stream is quite strong, a lot more powerful than it looks.
Enjoy the nearby hikes of Thunder Knob and Thunder Creek. Be sure to stop at Diablo Lake Vista Point for a beautiful overlook of Diablo Lake.
There is an active train track right near the campground. The train runs about 15 times a day and also throughout the night. Be prepared for the loud train whistles. I recommend bringing ear plugs in case you are a light sleeper.