It's a huge park, at least 10 blocks in length and width. There is ample parking underneath the park.
The park has an outdoor concert pavilion (all shows are free), a terraced garden, several large fountains, the huge Buckingham fountain, a playing pool, and of course the "Bean".
The concerts are great, often including famous and nationally touring artists, in addition to the Chicago Symphony Orchestra (one of the best in the world). The terraced garden can be relaxing if the weather permits, but often it is very hot in downtown Chicago. The fountains are very pretty. In addition you can see "the Bean" a mirror-shined metal sculpture which melts the sky with the earth you stand on. This very fun to see and play around.
There is also the Millennium Park walkway, which is a metal/architectural stairway/walkway.
Also try to see the Facial fountains, which I described above.