The Museum of Natural Science has exactly what you're looking for. The first time I came here, I fell in love with the place. Although it is geared more towards teenagers and adults, there are some things that kids will enjoy. They usually have very interesting IMAX shows and a planetarium. Admission is relatively inexpensive and is a great value for the exhibits they have. They host a variety of exhbits for a couple of months and they are usually very prominent and popular exhibits. I recently went and saw the Lord of the Rings Motion Picture Exhibit and was astounded by the intricacy and volume that the exhibit encompassed. I recommend this place if you are ever in search of a nice peaceful time downtown.
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