I wouldnt bother with the aquarium either. Seriously, a trip to the local pet shop to look at fish swimming in a tank would be much easier/more manageable!! And free
The zoo in DC is a lot of walking but so much fun. my 3yo loved it down there.
The last time I went to Reston zoo many years ago was nice, small, can pet animals, ride a pony, i think they have a hayride there too.
Leesburg park is awesome! a real up close view of animals, to the point you are walking alongside goats, llamas, pigs and other various animals in a large pen. There is a lake with fish you can see/feed and a wagon ride to see some of the bigger animals. Pony rides, and there was a camel ride too.
If you want to do something over a weekend (daytrip) go to Natural bridge to Virginia Drive Thru Safari. THAT was the best time we had when we went last august and we are planning a trip up there in the fall. The animals are friendly and come right up to your car and will even stick their heads in. Then there is a small walk around zoo area you can see other small animals and even get to feed a giraffe. For the entry fee, you can drive through the Safari as many times as you like. it was a lot of fun and we got many laughs out of it!!
here is a link to check out for more info
http://gettingoutofdcdaytrips.blogspot.com/2012/08/virginia-safari-park.html?spref=fb