Looks like most of them have been hit in one way or another but I would:
1) Go to Greenfield Village and the Henry Ford Museum
2) Go to the Charles H Wright African American History Museum. I still haven't done this myself but I hear it's phenomenal.
3) Campus Martius Park in Downtown is great when I usually visit (winter/Christmas) as they have ice skating. Not sure what happens in the summer. I've heard they do outdoor movies.
4) Have a Coney from Lafayette or National Coney Island (and tell me which you think is better). Personally I think they're each just as delicious as the MILLION other Coney Islands in the region, but it's fun to fuel the debate.
5) Greektown is great but my favorite spot in it is (ironically) Pizza Papalis. So deep, so delicious.
6) The Riverwalk Downtown is cool for me as it's new and a great change for the city but is probably not terribly exciting for tourists.
7) Royal Oak and Ferndale are great suburbs to hang out and eat in. It's also where (most) of the gay activity takes place. If you're feeling adventurous, check out the Menjos Complex in the city. I grew up in Detroit and it is no-kidding scary getting there, but it's a blast and surprisingly it draws gays from all over the region.
Detroit Institute of Arts (DIA) was great back in the day. I haven't been in quite some time so I can't vouch for it now but it still might be a good time.
9) Windsor. Because it's Canada and why not? All my friends and relatives insist we go everytime they visit just to say they've been to Canada but since 9/11 you've needed a Passport or Birth Certificate to get in and out.
10) Cider Mills. Not in season right now, but remember it for your next trip. Cider and donuts are awesome
In summary, Detroit may not be the most fantastic place to live and it certainly has it's share of problems (many of which represent widescale problems any given US city is only a stone's throw away from), but it's not as bad as people make it seem. At least not if you take the region as a whole.