Restaurant Overload

My daughter went to a restaurant yesterday evening. This may not mean much to most people, but it was very important for us. The last several times my daughter has been in a restaurant, it has led to a meltdown. She would gradually become more and more agitated, looking around wildly, reaching out to everyone, asking to be held, asking to be put down, moaning and humming, until one of us would leave dinner and put her in her car seat, where she would calm down immediately, and even fall asleep.
We don't know what it is exactly that bothers her so much, but we spend significant time trying to understand. Is it the lighting? Music? People? Conversation? Smells? Her own state of tiredness or hunger or pain? Is it just change? We don't want to subject her to what amounts to sensory torture, but we also feel like helping her to avoid any and all unpleasant experiences does not help her to learn to function in the non-autistic world.
Last night at the restaurant, she was not terribly happy, but she never got anywhere near meldown. My mom-in-law and I took turns giving her tours of the room every so often, and tried to distract her with table napkin peekaboo. We were able to finish our dinners, and the level of stress was fairly low.
What was the magic ingredient? Was she feeling better? It was pretty noisy, near the kitchen, so it can't have been the sound. If only she could tell us, we would do everything in our power to make her comfortable. We'll just keep trying to include her in family outings, and see if we can solve the mystery without damaging her.