Sometimes winter weather is just too extreme for frolicking outside. Escaping to a more comfortable environment may be the only solution. This time of year, indoor attractions in each of the 50 states offer families respite from the elements. Some are free or cheap, while many cost more than a night at the movies but provide a full day of entertainment, occasionally garnished with a dollop of learning. Attractions targeted at kids are cheaper all around — the youngest kids often enter for free — and seniors, children, and military typically pay reduced fees.