How much time to you want to spend up there and do you have anything you really want to do?
We have camped at many different places in the UP (mostly northern shore):
St. Ignace - state camp ground, visited Mackinaw City, Mackinac Island and St. Ignace
Brimley - state camp ground, went to the Soo. While the campground is nice, there wasn't a whole lot to do there.
Grand Marais - private campground, spent time seeing Pictured Rocks and doing lot of day hikes
Tahquamenon Falls - state camp ground, seeing the falls and there is a neat shipwreck museum at Whitefish Point.
McClain - state park just outside Hancock. spent time driving around looking at various water falls (there are a ton up there), the lake, and then spent time in Houghton downtown and MTU
Ft. Wilkins - at the very top of the UP, state park. spent time driving around looking at water falls, went to estivant pines (really, really old very large hardword forest - the trees are huge), and there is a fort to explore. Spent some time at the little lake there (Fanny Hoe) and Superior.
My dds, ages 12 and 9 now, liked various things at all of them. Where we go depends a lot on how far we want to drive - 3 hours vs. 7 hours, how long we want to stay and what we want to see.
One one trip we spent 3 nights at Fort Wilkins and 2 nights in McClain.
One trip we spent 2 nights in Grand Marais and 2 nights at Tahq falls.
One trip we spent 3 nights in Grand Marais.
One trip we spent 2 nights at Tahq Falls and 2 nights in Brimley. (prob the kids least favorite)
This August we spent 3 nights in McClain, but drove up past Ft. Wilkins.
And we have done two trips where we have spent 2 nights in St. Ignace (avoid the classic car festival weekend - that one was less than enjoyable at night).
Emily