Cosmic Cafe* first. You'll probably walk around the grounds and check out the views--wear yourself out a little. That is, if you drove up. Some people hike, so they come pre-worn out. I wouldn't recommend hiking on a hot day; the trail is long and (necessarily) uphill, and it doesn't have much shade.
The first thing you'll see upon your approach to the Mt. Wilson Observatory, no matter which way you arrive, is this impressive set of towers. They mostly serve Los Angeles-area radio and television stations. This view of them is from the Cafe.
Two of us had their excellent hot dogs. A third had the hummus and didn't finish it, so I finished it for her. Piggy.
Cosmic Cafe is popular and super-casual. Folks wear their hiking clothes. Dogs are welcome (there's a dish for them, too). Nothing fancy but the views.
*The Cafe is only open on weekends.