Jarling's Custard Cup is a Champaign favorite. I remember hearing that Tom Hanks once stopped by when I was a kid.
You can't go wrong here; everything is delicious. My two favorites are the banana split (fancy) and a single scoop of lemon on a sugar cone (simple). The Turtle Sunday is also fantastic :) In short, when in Champaign, it's always a good idea to stop by for a sweet treat!