I've always wanted to go to the Smithsonian. It would make a great field trip too. But I live in California, and the expense is not really justifiable for a field trip. ;)
It's a good thing they have a website!
The website for the Smithsonian is a great educational stop for classroom use. You can tour the museum room by room. You can also visit past exhibits, or you can go to the display room of choice by clicking it on the map and following the directional arrows.
Here's a shot of African mammals:
Or maybe your class is exploring space:
Navigate zooming in and out on displays (although you can't read the plaques). You can also control whether you're looking up or down, left or right.
This could be a great website to find a picture of an artifact you're trying to explain to students.
It's also for desktops or mobile devices!
Check out this free resource for yourself!