I just stumbled on a really cool web-based activity that has apparently been around since 2005 or maybe earlier. It's called Virtual Astronaut, and it puts you inside a 3D model of the ISS. You can fly around and locate various activities, which are brief educational modules on subjects like bones, muscles, plant growth in space, recognizing Earth features from space, and others. Most of them are biological or medical in nature, relating to the challenges faced by humans living and working in space.
Each activity ends with some sort of quiz or puzzle for which you gain points. Gain enough points and you can go to the space suit fitting area to purchase the components and assemble your space suit, and then buy 2 minute blocks of EVA time. Go to the airlock, press O to open the hatch, and you can then go outside and explore the 3D exterior of the ISS. There are no EVA activities other than floating around and looking, but it's still pretty cool.
Most of the activities seemed suitable for elementary school kids, though the ones involving muscles, bones, and the inner ear had some tough vocabulary to learn (the activity itself teaches you the vocabulary, and you can repeat each activity as often as you like to increase your score).
This "learning environment" was probably developed for NASA by Tietronix, though I can't see an "about" page for background so I don't know. It's well designed and easy to navigate. It requires downloading and installing a couple of plugins including one for a 3D viewer (though there are Flash-only and text versions also available).
I think this would work really well with Marianne Dyson's book Space Station Science which I have mentioned in a past post, providing a way for students to explore the ISS after they read about it (the book also has various activities).