Its a lot of fun, but like he said, I would plan to do other things around the area. The whole west coast of Michigan is gorgeous and has a lot of picturesque boating towns. Silver Lake is a blast, and my tip is to hit the dunes right after a rainstorm because there's typically a lot less people on them and the sand is packed down a bit more allowing for more speed. The JK is able to make every ascent on every dune, even though test hill looks scary and so do some of the others. Here's a cool pic from one of my times there...
When the sand is dryer, the top of the dunes have extremely loose sand and you'll want to keep your wheel speed pretty consistent without pegging the gas too hard. You don't want to air it out, but you also don't want this to happen:
It took us about an hour to dig out from that.
This pic will give you a little bit of an idea of how big the dunes actually are:
But if you're looking for stuff to do for an entire week, I would recommend checking out some of the coastal towns along the western coast. I enjoyed Sagautuck, Pentwater, Grand Haven, and a couple others. You might want to also check out Sleeping Bear Dunes where you can't drive but there's the most gigantic and steep dune I've ever seen with my own 2 eyes.
And if you're looking to wheel in MIchigan, you've always got Rocks and Valleys and Bundy Hill, my 2 favorite offroad parks in the state.