Okay... I'm biased. I spent my high-school years in the Detroit/Windsor area, and there's not much to recommend about the place. Windsor has a very large number of nudie bars, and the drinking age is lower. It has a casino. The only cool thing about Detroit that I remember is Greektown. There's a casino around there, now, too (IIRC -- the Windsor Casino was siphoning off too much cash). Joe Louis arena is neat from a historical perspective, if you're a hockey fan, or if there are any concerts going on. Depending on what "early" is (i.e. July 1st-4th), then the International Peace Festival is really cool, with lots of fireworks -- it's a big combined Canada/US birthday party. If you're into museums, there's the Motown museum (never been there), and the Henry Ford museum/Greenfield Village (which can be a complete day or more, alone).

Worthwhile day trips from the Detroit area include Cedar Point (in Ohio), and Toronto (okay... this ought to be more than a day trip).