I am a big smoothie fan though I have not worked in the loop for a couple years. When I was last in the loop at Wacker and Michigan my daily stop was Brian's for quality smoothies and juices. The smoothies are generally fresh fruit, sometimes frozen, and you can request fruit only as well as additives. If you go regularly they have punch cards for a free 10th smoothie or something along those lines.
Brian's Juice Bar & Deli
(312) 332-3435
80 E Lake St
Chicago, IL 60601-5900
You can also check
google smoothie search which lists bunches of smoothie options.
Lastly, If I had to pick the single best option it would be to take the river taxi to Joy Yees in Chinatown for a great bubble tea and an amazing round trip.
“Statistics show that of those who contract the habit of eating, very few survive.”
George Bernard Shaw, Irish playwright (1856-1950)