You can walk on the overhead bridge (free) to Mud Island but the tram is more fun.   Admission to the Island is free, however, the museum does cost.

The entire Mississippi River is designed out to scale at Mud Island.  You can start at the top of the headwaters and walk yourself all the way to the Gulf in a matter of 20 minutes.

Scale replica of the Gulf.  While you can wade your feet in the Mississippi, there is no swimming allowed in the Gulf. :)

Inside the Mud Island Museum.