There is no "best" coupon for a Science Center because they are only loosely affiliated - a promotion accepted at one would not be accepted at another. For example, the Carnegie Science Center has a relationship with Major League Baseball's Pittsburgh Pirates, but the one in Orlando does not.
Retailmenot is a good place to look for discounts for Science Center admission - just be sure to look at the right museum. Retailmenot's offers can expire or be otherwise invalidated, however, so they may not always work. Members of individual Science Centers receive free admission to that location as well as others at least 90 miles away from their home location. ASTC.org has a PDF file of all affiliated Science Centers participating in the promotion. Some Science Centers, such as the California ScienCenter, are simply free to the general public without any coupon or membership.