Biologists, ecologists, gemologists, archeologists, and science buffs of all kinds will be able to spend hours in the United States National Museum of Natural History. As with the other Smithsonian Museums, admission is free! The down side to that is you may have to put up with some crowds or school groups, but it is worth it.
The bottom floor eases you into the amazing collection of natural pieces. Many species of mammals, reptiles, amphibians, and birds are on display in the first room. There are interactive displays to keep the interest of the young scientist, and more information than any visitor can take in on a first visit.
While you are there, visit the planetarium, see a tyranosaurus replicated from bone, view beetles that look like jewels, and perform interactive studies.
The crowd favorite and main attraction is always crowded and heavily guarded -- the jewel display room, which contains the The Hope Diamond, a huge blue diamond set in a white diamond necklace, that absolutely takes your breath away.
Great displays. Great fun. Floor after floor of awesome, inspiring, and educational resources.