Groupon: The Children's Museum of Houston for $8, you get two admissions to the Children's Museum of Houston (up to a $16 value). They expire October 20, 2011.
From what I understand in the comments section, if you purchase 2 at one time (for example), paying $16 total, they must all be used at one time, so 4 admissions total. But if you purchase 2 separate Groupons (buy one, check out, pay. Come back, buy one, check out, pay.), they may be used at separate times (2 admissions one day, 2 admissions another day).