My family has been visiting Multnomah Falls for years, as it's just a quick 30 minute drive East of Portland.
There's no admission fee, there are bathrooms available on site, and in case you are hungry and have time, there is a restaurant. The 1 mile switchback trail to the top is paved, and leashed pets are welcome. And if you make the effort, at the top there is a wooden deck extending over the falls rewarding you with incredible views.
The hike up is a great workout, and though it can get crazy busy in the summer, the view from the summit is well worth the effort.
If you expect to pass by be sure to stop, even if you only have a few minutes. You don't have to hike to the top to enjoy the view - you can get a beautiful shot of the falls from the great stone bridge right at the base.