If you go up to Hart's Pass, make sure you drive all the way to the top of Slate Peak. There is an amazing view from up there. Oh, and you'll want to bring plenty of mosquito repellant in August.
Also, Winthrop is a scenic little town just down the road that has some nice shops and restaurants. I've heard there are good campsites in the area, too.
If you travel farther west on SR20, up into the Cascades, there are a couple other fun places to visit. One of those places is Diablo Lake. There is plenty of hiking to be had around the lake and you can even go on a boat tour on the lake. You have to drive across a very tall dam in order to get to the boat tour and some of the hiking trails.
If you drive another 15-20 minutes west of Diablo Lake, you come to the town of Newhalem. It is completely owned and run by Seattle City Light. They are responsible for managing three hydroelectric dams in the area. They have a small visitor center in the center of the town, across from the general store, that is staffed by a ranger who can tell you plenty of info on the area.
There is a pedestrian bridge in the town that spans the Skagit River and leads to a short walking trail called the "Trail of the Cedars." That trail is less than a 1/2 mile loop, and on the far end of the loop is an old mill, that, I believe, is now a historical site.
On the east side of the town, there is another pedestrian bridge that spans the river and leads to the Gorge Power Station. During certain times of the year, you can see thousands of salmon in the river from that location. At the rear of the power station, there is a door that leads to another visitor center that displays old photos that show the history of the construction and operation of the dams. You are also able to view the power generating station in operation from up there.
There is a short walking trail behind the Gorge Power Station there that leads up to Ladder Creek Falls. In the summertime, there are colored lights on falls at nighttime. It's kinda nice.
That's probably way more than you wanted to know, but if you make it that far up the Cascades, it's a nice place to visit. And of course, there are tons of hiking trails and campsites in the area.
David