Let's get cost out of the way. It's 7 USD (per person) for the Samuel Oschin Planetarium show. Got that? OK. The rest is free.
Free! In Los Angeles? Yes.
Bring your camera and binoculars if you have some. You will be treated to some great views of the L.A. area. You might finally say, damn, this place is big! On a clear day you can see down to Santa Monica and then down to Palos Verdes.
The Observatory is a fun and interactive experience for "kids of all ages". From the novice to the astrophysicist there is something for everyone. Hang around and early for the Tesla Coil exhibit.
Most of us in Los Angeles like to drive to the supermarket and park right next to the handicap parking. So, keep that in mind when reading some of the reviews here. Parking can be a challenge if you arrive late in the morning. No fear, there is parking along the roads and if you are European you will feel right at home. Park anywhere you can (with limits of course and safety).
Besides what is inside the observatory there are plenty of hikes and trails to do. Bring water and snacks and a backpack. You will have fun!
Then of course there is the draw of the Hollywood sign and that this Park has been used in a variety of films and TV shows. I could list them, but that's why they work so hard at Google. Keep them busy!
Get there early and enjoy some free* entertainment.
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