A children-oriented science museum on the riverfront, with hands-on physics, biology, and engineering exhibits, an IMAX dome theatre, and a Power of Play hall aimed at younger kids. Reasonable rather than spectacular by big-city standards, but the best rainy-day destination in town if you are travelling with children.
The same building has had several near-closures and re-openings; check current hours before you go. The IMAX is often the strongest reason to visit.