This big, bright, family-friendly eatery is an unusually decent (scratch that and make it: very good) museum cafe.  They offer a wide range of sandwiches and other reviving snacks. You may have to wait in line, but it will still be faster than exiting the museum and coming back, so try to be patient. In summer, you can eat outside in the amphitheater by Bush City, a concentrated planting of native shrubs and trees.