Apologies to all members of the Marine Corps for the cheesy title. Couldn't resist.
My parents, Grandpa, the Wife and I had a wonderful trip to the US Marine Corps Museum located in Quantico, VA. It's been on my List of DC Sites To See for a while, and when I saw it got rated as one of the top 10 things to see in Northern VA, I was sold.
The museum itself is larger and more complete than I imagined. WW II, Korea and Vietnam are all covered in depth - with plenty of hardware to be impressed with. I even found the mannequins in the dioramas to be eerily lifelike.
Even if you're not a fan of the Marines or blowing stuff up, it's still worth checking out just for the interesting historical perspective.
It's also worth noting that it's both free, and with the wheelchair they provided for grandpa, quite senior friendly.