I drove around Lake Michigan last September--I keep meaning to write up a trip log, but I never seem to quite get around to it.
Anyway: it's certainly possible to do it in 4 days. However, I'd recommend taking as much time as you can afford. I took 6 nights/6.5 days to do it (I was only along Lake Michigan itself for a few hours on the first day, so I only count that as .5 days), and wish I could have taken twice as long--there was so much that I would have liked to have seen that I had to pass up. But then, I was eager to drive into every nook and cranny along the lake--I could have easily trimmed a full day off the trip if I didn't insist on going all the way to the tip of the Door Peninsula in Wisconsin and back.
Can't help you with the camping and the whitewater rafting, as those aren't really my thing. Some things I did particularly enjoy were:
The two National Lakeshore areas along the lake:
Sleeping Bear Dunes National Lakeshore and
Indiana Dunes National Lakeshore. The latter of which I visited twice as it was my designated start/finish point.
Several of the state parks around the lake, which offered plenty of hiking. I enjoy hiking very much as long as it's not too difficult (I'm not in that great shape) and it's only for a few hours (I'm no all-day, let alone multi-day hiker). The state parks offered plenty of this. I can now say I've walked on dunes in all four states bordering Lake Michigan. (Yes, surprisingly there is still undeveloped land on the Illinois shore of Lake Michigan, in
Illinois Beach State Park.)
In the upper peninsula I went into both Stonington Peninsula and Garden Peninsula:
http://chuckcarroll.smugmug.com/photos/26318457-L-1.jpg
You can drive all the way out to the tip of the Stonington Peninsula, where there's a small park and lighthouse. (The lighthouse itself is not all that special; the "house" part is long since gone, leaving only the tower, and the lens and light are no longer there; but you can climb the tower and have a nice view of the lake from there.) The best part of this, though, is the drive to the lighthouse itself; the last mile is a gravel road, closely bordered and covered by trees, such that in most places it's only wide enough for one car to pass. Every few hundred yards it widens so that cars going in opposite directions can pass each other; but if two cars meet outside of these spots, someone has to back up!
On the Garden Peninsula I visited
Fayette Historic State Park, which features a former iron smelting town which has been turned into something of a historic museum and park--see how people lived in a late 19th century company town. Very neat. (The website calls it a "Ghost Town," but I don't think that's appropriate, as "Ghost Town" conjures up images of a completely abandoned and decaying town. While no one lives here anymore, the buildings are mostly well-kept and several of them have exhibits inside--more "Museum Town" than "Ghost Town" to me.)
However, I would note that it's not worth driving
all the way to the tip of the Garden Peninsula--there's not much to see there, all privately owned land, and not even really a good place to stop and take a picture.
If you're into wines, you can find wineries all over Wisconsin and
Michigan, but the best ones (and also the highest concentration of them) are the ones on the Old Mission and Leelanau Peninsulas.
There's a lighthouse and beach at the very tip of the Old Mission Peninsula. The waters around the beach are very shallow and you can go quite a ways out into the lake here; there's even a sandbar stretching out from the beach (a bit east of the lighthouse) where you can walk quite a ways out into the lake (at least 1500 feet, I estimate) without ever going in water deeper than a few inches. When I was there, someone had made a large pile of rocks (6 or 7 feet tall) near the end of the sandbar.
The highlight of my trip was walking across the
Mackinac Bridge, but you can only do that on Labor Day.
