I can't speak to the NDU internship, but I worked on Fort McNair for several years. It's a nice, quiet installation. Presidents HW Bush and Clinton regularly went there to jog. Some will tell you that the local neighborhood is not so great, but I went running off the installation almost every day and never had any difficulties (it's an easy jog to the Jefferson Memorial and Tidal Basin). It's relatively easy to drive to and there is a Metro stop (Waterfront) about a half mile away. It's a green and pleasant piece of DC.