You'll need a car. Snoqualmie Falls is nice, although you're visiting at a low flow period. Peak is usually during December/January when it rains a lot more.
Mount Baker is nice, although your car will get broken into and everything inside stolen when you park it at the trailhead and go for a hike. DON'T FUCKING LEAVE ANYTHING INSIDE TO STEAL.
Hurricane Ridge in Olympic National Park is nice. Plus you can take a couple of ferry rides to get over there. Port Townsend is a pretty little town if you want to stop for dinner on the way back.
Mount Rainier and Mount St. Helens are also great for nature.
If you're driving up from (or down to) California and have an extra day, stop at Crater Lake. It's a long detour off I-5, though. If you try to go there from Seattle, it's a full day's trip each way.