Two biggest days of the year for museums are the day after thanksgiving and day after Christmas. When I worked in museums people who had guests in town took the opportunity after the holiday to show their guests around. No avoiding crowds. Best you can do is go first thing at opening. That way you’ll be there as the crowd builds as opposed to arriving to a crowd that is getting worse and worse. Alternative to Smithsonians is to go to the smaller historic houses and galleries. Many are open and just less popular so less crowded.