Should I walk, drive, or take transit to get around downtown Chicago?

I don't mean getting there in the first place, but getting around. If I plan on, say, going to the Museum Campus, Sears/Willis Tower, and Magnificent Mile in the same day, then is it feasible to just walk? I could take the CTA but I have 3 people in my party so it costs $5.75 for each one-day p, and I have to go to one of the two welcome centres to get one (stations only sell Transit Cards). Or should I park at each place?