Nordita is an interesting place. The building is located right next to the Department of Physics, at Stockholm University that I had visited before. Nordita was founded in 1957 in Copenhagen, Denmark, and was relocated to Stockholm in January 2007. Their building is shown in the photo above. If you have been there before, the entry to the AlbaNova University Center is to the left. You can find some more photos of the building here.
Nordita's main mission is to strengthen the collaboration in theoretical physics between the five nordic countries Denmark, Finland, Iceland, Norway and Sweden. In their logo, shown to the left, each triangle stands for one of these countries. Nordita's research areas cover quite a quite diverse range of topics in theoretical physics: astrophysics and astrobiology, condensed matter, statistical and biological physics and high energy and nuclear physics. Compared to Perimeter Institute, it is a somewhat smaller institution. Though the faculty is of similar size, they have considerably fewer postdocs and students. Nordita further has a very active scientific program which brings together groups of leading experts to work on specific topics for extended periods, similarly eg to the programs at the KITP.
If you have not visited Stockholm before, let me add it is on my list with the most beautiful cities I've been to (together with Cape Town, Vienna and San Francisco). I don't speak Swedish but it is remarkable that literally everybody in Stockholm seems to speak English (and isn't funny about using it either as eg the French are). If you know German and English you can guess a lot of Swedish words though. Altogether a very interesting and enjoyable stay. Minus the snow.