It costs over $20 a head to get in, but I really dig the Boston Museum of Science's Butterfly Garden. Yeah, I'm a dork, but those little buggers are just so cute. Plus they can, and will, fly right over and land on you, get comfortable, and stay a while.
Check out this pic of one on my wife Lisa, who then crawled onto my finger for a few minutes, too (below). That is the same butterfly...that species has iridescent blue wings on top, and drab brown on the bottom, to confuse predators when they're flying. Poor little fellah had some wing torn off (that's common...the wings are super-fragile and butterflies don't live more than a few weeks) but it didn't seem to hamper his flying much.
If you go, make sure it's when the sun is still shining brightly...most butterflies are not nocturnal and they won't fly much as dusk approaches. However, I have been in there about an hour before closing, when it was about a half-hour after sunset, and while not much was moving...you could get really up-close and personal with many of them hanging on the branches. Plus the staff is happy to chat with you at length since it's often pretty quiet that late in the day; you get the place to yourself! Still, daylight really brings the colors out so I do recommend taking a weekday to head over if you can (weekends are pretty mobbed).
I also overheard that any chrysalises that're ready to hatch, tend to do so around noontime. They don't mind you hanging out in there for quite a while, but the space isn't very big...there's enough room to comfortably allow perhaps a dozen or so people at most. And of course, like the rest of the MoS, there's often little kids galore in there...again, especially on weekends.