Get a chance to see the night sky up close and personal at the Coit Observatory. Each (weather-permitting) Wednesday, Boston University hosts a Public Open Night, giving you have access to their telescopes to stargaze, check out planets, and see the moon. Staff members are there to offer you educational tidbits and provide any help you need. These events are fun,free, and educational so this is a great idea for kids or out of town guests.
Be sure to check their twitter feed the day-of, to make sure the event isn't canceled due to weather.
You can also give them a call at (617) 353-2630.