Most people on a transit that have been to MSP before just head to the Mall of America to burn some time. Heading into downtown MSP works, but eats up more time. Also, downtown MSP is mostly offices and some hi-rise residences; and besides some shopping and recognizing the benefits of being able to walk indoors across a large section of the commercial area downtown (i.e., via the skywalks) there's not much that would catch my fancy during 90-105 minutes downtown (especially given you'll need at least 75 minutes for transport roundtrip, perhaps should budget 30-45 minutes to clear security c. 15 minutes before departure and will need to get oriented to walk downtown and see much of anything). If the transit time was 6 hours, I'd be more comfortable heading downtown to explore (if I didn't know the city).