There's nothing like climbing a volcano with a volcanologist.
Mt St Helens is a hike broken into three distinct parts:
The peaceful stroll through the woods.
The hand-over-fist scramble over rough (and sometimes sharp) boulders... swap out your hiking poles for leather-palmed gloves in some places).
The long slog up the biggest sand (actually ash) dune you've ever seen in your life, where every step forward includes a half-step slid back.
This is the ONE hike where the summit is absolutely worth it. Since the hike comes up the back side, it means you don't get to see the view into the crater until you get to the edge. Just be careful to not get too close... lest the volcano make lunch of you.
Also, you can do this hike in a day... but it's a hard day if you don't get your ass out of bed on time. While the sunset is lovely, it's not comforting to know that you've got to find your way down in the dark.