Find someone who will walk you through the aquarium! We go during the afternoon on school days - the field trips are gone by then - and if you get someone who will walk it with you, or at least find someone near each exhibit, they will show you things you would normally miss
If you want to do DC, just focus on a couple of things for 2 days and don't try to do it all. Heck, we spent 2 full days at the Smithsonian and didn't see 1/20th of it
