If you are going to the Columbia River Gorge (e.g., for waterfalls and hiking), you can cross over to Washington and hike up Beacon Rock, which is a neat hike and a beautiful view at the top on a relatively clear day.

If you just want to do something in Washington, you could explore downtown Vancouver and the Vancouver waterfront.

Everything else is either, or both, a bit farther away (e.g., the Coast) or closed for the season (e.g., Ape Cave).