The Mpls/St. Paul airport is the headquarters for Northwest airlines (which I happened to work for before). Alot of people come to here on plane. Personally, I would rather chose doing a road trip because I love to see everything.
If you really don't have a transportation of your own when you are in St. Paul..that is fine. There is a big bus system in the Twin Cities. There is many people that actually choose to take a bus to where they need to go. The only time I don't suggest that is if you don't like the cold. I never liked standing out in the cold waiting for a bus, but now I even started to notice that some of the bus shelters have something to where you can push a button and a heating light turns on.
The areas around St. Paul are fairly easy to navigate with a good map as most roads are well marked and go north/south or east/west. Downtown is a mess, as the road scheme doesn't really make any sense. Make sure to have a map and ask directions. The best way to get around is by car, but there is a decent bus system (see link).
You'll be doing a lot of driving in St. Paul. The Twin Cities of Minneapolis/St. Paul has a lot of highways so get a good map.