Just for the heck of it, I looked at using the nwa.com multi city option in Jan 2003. Using a routing YWG/LAX via MSP, LAX/DTW then DTW/YWG via MSP I saw a fare of $542.71,
I used Jan 10 to 15 so there would be a Saturday night stay.

This means you go to LAX, wait a few hours, red eye to DTW, a few days in DTW, DTW back to YWG via MSP. You can play around with this function using your required dates to see what works best.

Generally If I want to rack up miles I go from ORD/MSP/LAX on Saturday afternoon, have a nice dinner at Encounters located in the LAX theme building, take the red eye around 1:30 am LAX/MSP then MSP/ORD Sunday am. With a 14 day advance purchase, and this qualifies for a Saturday night stay even though its only a few hours, the fares have been around $200.00.

Good Luck!