Visit to housing museum of natural science

I would like to visit HMNS on Dec 30. Is it necessary to buy tickets in advance or can I get them when I arrive? I don’t mind buying the tickets in advance but I need to select a time and that’s what I’m unsure about. Is it pretty strict about the times? Do I need to show up at the exact time I select? Or can I show up after the time I selected? I would like to head to the museum straight from the airport in a rental car. Is it smart to leave my luggage in the trunk? Does the museum have a parking garage?