MSP is one of only two or three airports nationwide that have re-installed the self-service luggage lockers airside. They're all over the terminal area (one at beginning of F, one about 3-4 gates down on E, one midway across the C/G concourse connector, probably a few other locations I've forgotten). They're pretty cheap, definitely secure (fingerprint scan required to rent initially and to unlock), and accept all major credit cards for your points earning pleasures. I would not hesitate one bit to leave my luggage in one (or more) of these lockers and head out on the town.

Don't get me wrong, MSP is quickly becoming a very nice airport to lay-over in with capital improvements such as food, retail, and creature comforts. But with a layover that long, I'd really recommend some fresh, albeit frosty, Minnesota air.