They will be crowded but not summer tourist season crowded. You might want to avoid or quickly go through choke points like the area around the Hope Diamond at Natural History. The Objects of Wonder exhibit is usually not crowded even though it is a wonderful mix of items because so many people are in the Gem & Mineral Hall. Same with the others - the museums are big enough you can find areas where you will be more comfortable. American Indian has a gorgeous exhibit of Preston Singletary glass that you shouldn't miss.