I did same thing a week ago,, as part of DTW-MSP-NRT.

I had 5hours too, and went to Mall of America.

First, I used famous airside locker @ MSP
-It was very large enough to hold my carryon and laptop case.

Second, I used light rail to MOA.
-They sell tickets, but there is no check gate (like Germany subways...)
just buy one they are relatively cheaper than NYC.^
It takes 10 or so minutes to get there by rail.

I was surprised by number of Caribou(sp?) Coffee in there and size of the mall.
(is it the laregest in the states?)