San Diego Safari Park's Butterfly Exhibit is worth planning for - it's only open for 1 month or less out of this year. We went this year for my mother-in-law Helen's 60th Birthday and she described it as "a piece of heaven"
Brookside Gardens in Wheaton, MD. Indoor butterfly exhibit and beautiful gardens including water garden, Japanese garden, children's garden, a labyrinth, and much more. The gardens are free. Butterfly exhibit requires admission fee. Bring a picnic. Beautiful place!