livingsocial has this deal from Seven Peaks Duneland of Chicago:
1-Day Waterpark Pass, Year-Long Pass, or 1-Year Pass Extension

• $10 ($59 value) for a 2015-16 Chicagoland Pass of All Passes
• $5 ($14 value) for one waterpark all-day pass
• Current passholders: you can extend your pass for another year with this deal
• Waterpark features 70-feet free-fall drop slides for some adrenaline-pumping fun