The Griffith Observatory in Los Angeles, California is cool. Since their remodeling, a timed ticket system is in place and reservations are required. No parking on site so the reservation is for the bus transportation to get there.
The Observatory is perched above the valley with a great view of the Hollywood Sign. Exhibits explore space and our place in it. I loved the swinging pendulum and the Tesla coil that produces lightning! A Zeiss 12-inch refracting telescope is available if you go in the evening and during clear days Solar Telescopes are found in the Rotunda. One of the highlights is the state-of-the-art Samuel Oschin Planetarium that creates multi-media star shows. Also, the new Leonard Nimoy Event Horizon features a short film by the actor himself. Make sure to save room to eat at The Cafe with outdoor seating overlooking the valley. Some effort and planning is required to visit here but it's well worth the effort.